architects FROM LONDON Hidden Building Costs
What Are the Hidden Costs of Building Work?
↓
RIBA Chartered
Award winning
London · Vienna · Rome
Featured in The Times
Panel at Grand Designs Live
Hidden Building Costs intro
What Are the Hidden Costs of Building Work?
When homeowners budget for an extension, renovation or conversion, the obvious figure is usually the builder’s quotation.
But the builder’s price is rarely the whole project cost.
There can also be costs for:
- Planning
- Building Control
- Architectural drawings
- Structural engineering
- Party-wall matters
- Scaffolding
- Waste removal
- Temporary accommodation
- Finishing
- VAT
- Unexpected defects
- Changes during construction
The Federation of Master Builders specifically identifies professional fees, planning and Building Control, Party Wall costs, scaffolding, skip hire, temporary living arrangements, final finishes, VAT and contingency among the expenses homeowners most commonly overlook. FMB
At Tholos Architects, we encourage clients to think about the whole project budget, not simply the amount written at the bottom of the builder’s quote.
hidden building Cost explained
Builder Quote vs Total Project Cost
A builder may quote:
£80,000
for the construction work.
That does not necessarily mean the project will cost £80,000.
The total budget could also need to cover:
- £4,000 architectural and technical work
- £1,500 structural engineering
- Planning and Building Control fees
- Party-wall surveyors
- Additional kitchen upgrades
- Flooring
- Landscaping
- Temporary accommodation
- VAT if not already included
- Contingency
The finished project could therefore cost substantially more than the initial construction figure.
This does not necessarily mean the builder has hidden anything.
It often means different costs sit outside the builder’s scope.
The Most Common Hidden Building Costs
The items most frequently forgotten include:
- Professional fees
- Application fees
- Structural calculations
- Building Control inspections
- Party-wall costs
- Scaffolding
- Waste
- Temporary works
- Utility alterations
- Kitchen and bathroom allowances
- Flooring
- Decoration
- External works
- VAT
- Contingency
The best way to avoid a budget shock is to identify them before construction starts.
Architectural Fees
Architectural fees may sit outside the construction budget.
Depending on the project, architectural services can include:
- Measured survey
- Existing drawings
- Design development
- Planning drawings
- Planning application
- Building Regulations drawings
- Technical drawings
- Construction information
These costs need to be accounted for separately unless specifically included elsewhere.
Structural Engineer Fees
Structural engineering is another commonly overlooked professional cost.
You may require calculations or drawings for:
- Steel beams
- Structural openings
- Foundations
- Loft floors
- Roof alterations
- Columns
A single calculation can be relatively inexpensive, while a complete structural package for a complex extension can cost considerably more.
Planning Application Fees
Planning application fees are separate from the cost of preparing the architectural drawings.
For England, the Planning Portal currently lists the fee for a typical householder planning application at £548. Wales has a different fee structure, and Scotland and Northern Ireland also operate their own systems.
Depending on the project, you may also need additional reports or supporting information.
Lawful Development Certificate Costs
Even where planning permission is not required, homeowners sometimes apply for a Lawful Development Certificate to obtain formal confirmation that the proposal is lawful.
This involves a separate application and fee.
That cost is easy to miss if the original assumption was simply:
“It is permitted development, so planning costs nothing.”
Permitted development may reduce the planning process, but it does not necessarily eliminate every planning-related cost.
Building Control Fees
Building Control is another cost that may sit outside the builder’s quotation.
Current 2026 guidance gives indicative costs of:
- £250–£550 for a single-storey extension full-plans application
- Around £330–£400 for related inspections
- £250–£300 plan charge for a two-storey extension
- Around £370–£470 for inspections
- Approximately £260–£360 plan charge for a loft conversion
- Around £160 for loft-conversion inspections
Fees can vary substantially between authorities and providers.
Party Wall Surveyor Costs
Party-wall costs can be one of the more expensive surprises.
They may become relevant where work involves:
- Excavation close to a neighbouring property
- Party walls
- Shared structures
- Loft structural work
- Boundary construction
Current 2026 guidance puts:
- A Party Wall Award at around £1,000
- A single surveyor arrangement for an extension at around £1,200–£1,500
- Two-surveyor arrangements for an extension at around £2,400–£3,000
Costs can rise further in more complicated disputes.
If the adjoining owner consents to the notice, a surveyor may not be required, so the actual cost depends on the circumstances.
Planning Consultant Costs
Most straightforward domestic projects will not necessarily require a planning consultant.
However, specialist planning input may be useful for:
- Difficult sites
- Previous refusals
- Conservation areas
- Listed buildings
- Unusual development
- Planning appeals
This would normally sit outside standard architectural and construction costs.
Specialist Survey Costs
Some projects require additional investigations.
These might include:
- Structural survey
- Drainage survey
- CCTV drain survey
- Tree survey
- Ecology survey
- Asbestos survey
- Ground investigation
- Heritage assessment
Whether these are required depends on the property and proposal.
A realistic early budget should allow for the possibility rather than assuming none will be necessary.
Drainage Costs
hidden building Cost
Drainage Costs
Drainage is frequently underestimated.
A project may require:
- Moving drains
- Replacing old pipework
- New inspection chambers
- New rainwater drainage
- Soil-pipe alterations
The problem is that drainage routes are not always clear until:
- Surveys are completed
- Trial holes are opened
- Construction starts
This makes drainage one of the most common areas where unexpected work appears.
Build-over Agreements
Where building work affects certain public sewers or drains, additional consent may be required.
This can introduce:
- Application costs
- Survey requirements
- Design changes
- Additional construction requirements
Drain ownership and position should therefore be checked early.
Ground Conditions
Foundations are often priced on assumed ground conditions.
Once excavation begins, the contractor may discover:
- Soft ground
- Made ground
- Deep existing foundations
- Tree-related conditions
- Unexpected services
That can mean:
- Deeper excavation
- More concrete
- Different foundation design
- Additional engineering
This is one reason foundation costs should never be treated as completely fixed before the ground is opened.
Existing Building Defects
Renovation projects can expose problems that were hidden behind finishes.
These might include:
- Rotten timber
- Damp
- Cracked masonry
- Failed lintels
- Poor previous alterations
- Inadequate wiring
- Leaking plumbing
FMB specifically recommends additional contingency for older properties and structural alterations because hidden issues are more likely to emerge during the work.
Asbestos
Older buildings may contain asbestos in materials such as:
- Textured coatings
- Cement products
- Pipe insulation
- Floor tiles
- Roofing products
Where asbestos is suspected, appropriate surveying and specialist removal may be required.
This can affect:
- Programme
- Demolition
- Cost
It should not simply be demolished with ordinary building waste.
Scaffolding Costs
Scaffolding can be a substantial project cost and may not always be included in initial discussions.
Current 2026 guidance gives:
- Around £20–£25 per m²
- Approximately £875 for a typical two-storey semi-detached house setup
- Around £550–£700 for some smaller single-scaffold arrangements
Costs increase with height, access difficulty and duration.
Always check whether the builder’s quotation includes:
- Erection
- Hire period
- Inspections
- Dismantling
- Any required permits
Extended Scaffolding Hire
Even where scaffolding is included, the price may only cover a defined period.
If construction is delayed, additional hire may apply.
Potential causes include:
- Weather
- Material delays
- Planning changes
- Structural discoveries
- Contractor programme changes
This is worth clarifying before work starts.
Temporary Roof Costs
Major roof alterations and loft conversions may need a temporary weatherproof structure.
This can add considerably to the scaffolding budget.
It may nevertheless be worthwhile because it reduces the risk of rain damage once the existing roof is opened.
Skip Hire and Waste Removal
Waste disposal is another easy cost to underestimate.
Current 2026 guidance gives approximately:
- £75–£175 for a small 2–3 yard skip
- £130–£320 for a 4–5 yard skip
- £190£450 for a 6–8 yard builder's skip
If a skip sits on public land, a permit may add approximately £15–£60.
A large renovation may require several skips.
That cost can quickly become significant.
Specialist Waste Disposal
Not everything can go in a normal skip.
Separate arrangements may be needed for:
- Asbestos
- Contaminated material
- Certain electrical items
- Refrigerants
- Hazardous materials
Specialist disposal usually costs more.
Demolition Costs
A quote may appear to cover construction while leaving strip-out or demolition partly excluded.
Demolition can include:
- Old kitchen
- Bathroom
- Floors
- Internal walls
- Existing extension
- Roof
- External paving
Check whether this work and the waste it produces are included.
Temporary Works
Some structural projects require temporary support before the permanent structure can be installed.
Examples include:
- Acrow props
- Needles
- Temporary steel
- Temporary walls
- Weather protection
These costs may be substantial on projects involving large structural openings.
Steel Price vs Steel Installation
A quote for a steel beam is not necessarily the complete structural cost.
You may also need:
- Fabrication
- Delivery
- Crane or lifting equipment
- Labour
- Padstones
- Columns
- Connections
- Fire protection
- Making good
The beam itself may represent only part of the total cost.
Crane and Lifting Costs
Large steel beams, glazing panels or other heavy components may need specialist lifting.
This can involve:
- Crane hire
- Road access
- Parking suspension
- Lift planning
- Additional labour
Access should be considered while the project is being designed.
Parking Suspensions and Permits
Urban projects may need:
- Parking-bay suspension
- Skip permit
- Scaffold permit
- Road occupation arrangements
Individually these may not be major expenses, but together they can add hundreds or thousands of pounds depending on the project duration and location.
hidden building Cost
Difficult Site Access
Restricted access can increase labour considerably.
Problems may include:
- No side access
- Terraced house
- Narrow streets
- Limited parking
- Materials carried through the house
This affects:
- Delivery
- Excavation
- Waste removal
- Concrete
- Steel installation
A project with difficult access may cost noticeably more than an identical design on an open site.
Utility Alterations
Construction may uncover the need to move:
- Gas meter
- Electricity meter
- Water supply
- Consumer unit
- Boiler
- Drainage
Some of these works need to be completed by the relevant utility provider or specialist contractor.
They should not be assumed to be included automatically.
Boiler and Heating Upgrades
Adding floor area can increase the demand on the heating system.
The existing boiler may not be suitable for:
- Large extension
- Additional bathroom
- Underfloor heating
- Loft conversion
Possible extra costs include:
- New boiler
- Larger hot-water cylinder
- New controls
- Additional radiators
- New pipework
Electrical Upgrades
An extension may require more than adding a few sockets.
The existing electrical installation may need:
- Consumer-unit upgrade
- New circuits
- Earthing improvements
- Testing
- Certification
This can add cost beyond the room-by-room electrical specification.
Kitchen Allowances
One of the most common budget traps is the kitchen allowance.
A builder may include:
£10,000 kitchen allowance
but the kitchen eventually selected costs:
£18,000
That £8,000 difference is not necessarily a builder variation.
It may simply mean the original allowance was lower than the chosen specification.
Check:
- Units
- Worktops
- Appliances
- Installation
- Plumbing
- Plumbing
individually.
Bathroom Allowances
The same issue applies to bathrooms.
A quotation may contain provisional allowances for:
- Bath
- Shower
- WC
- Basin
- Taps
- Tiles
Higher-specification products can quickly exceed them.
Always understand the assumed allowance before accepting the overall price.
hidden building Cost
Prime Cost Items and Provisional Sums
Builder quotes may contain:
- Prime Cost items
- Provisional Sums
- Allowances
These are not necessarily fixed prices.
If the final product or work costs more than the allowance, the project cost increases.
This is particularly common for:
- Kitchens
- Bathrooms
- Tiles
- Groundworks
- Drainage
- Specialist items
Flooring
Floor finishes are frequently excluded from construction quotations.
A builder may provide:
- Structural floor
- Screed
but not:
- Timber flooring
- Carpet
- Tiles
- Luxury vinyl
Across a large extension or renovated ground floor, this can become a substantial additional expense.
Decoration
Another common exclusion is final decoration.
The builder may leave:
- Plastered walls
- Primed joinery
- Unpainted ceilings
rather than a completely decorated home.
Check whether the quote includes:
- Mist coat
- Full wall paint
- Woodwork
- Doors
- Ceilings
External Works
The project may finish inside but leave significant work outside.
Potential additional costs include:
- Patio
- Steps
- Drainage
- Paths
- Fencing
- Garden repairs
- Retaining walls
- Landscaping
Building an extension can destroy or disturb part of the existing garden, so the cost of making good should not be ignored.
Making Good Existing Rooms
Building work rarely stops neatly at the edge of the new extension.
Existing rooms may require:
- Replastering
- Flooring repairs
- Decoration
- New skirting
- Electrical alterations
This should be part of the budget
Matching Existing Materials
Matching an older property can cost more than using generic new materials.
Examples include:
- Brick
- Roof tiles
- Stone
- Timber mouldings
- Period doors
You may need:
- Reclaimed materials
- Specialist fabrication
- More labour-intensive installation
hidden building Cost
Temporary Accommodation
Large renovations can make the property difficult or impossible to live in.
Temporary accommodation may be needed where you lose:
- Kitchen
- Bathroom
- Heating
- Water
- Safe access
FMB lists temporary accommodation and storage among commonly overlooked renovation costs.
Storage Costs
Furniture and possessions may also need to be removed during construction.
Potential costs include:
- Storage unit
- Removal company
- Packing
- Insurance
A project lasting several months can make this a meaningful expense.
Temporary Kitchen and Bathroom
Even if you remain in the property, you may need temporary facilities.
These may include:
- Temporary sink
- Small cooking area
- Temporary shower arrangements
FMB includes temporary kitchen and bathroom arrangements among often-forgotten costs.
Security Costs
Construction can temporarily reduce the security of the property.
You may need:
- Temporary doors
- Site fencing
- Alarm modifications
- Lockable storage
This is particularly relevant where external walls or roofs are opened.
Insurance During Building Work
Standard home insurance may not automatically cover major construction projects.
You may need to:
- Notify the insurer
- Change cover
- - Obtain specialist renovation insurance
The builder should also have appropriate insurance, but that does not replace the homeowner’s own obligations.
Check this before construction begins.
VAT
VAT can be one of the largest apparent “hidden” costs.
Always establish whether:
- Builder quote includes VAT
- Professional fees include VAT
- Supplier prices include VAT
For example:
£80,000 excluding VAT
and
£80,000 including VAT
are very different budgets.
Never compare quotations without checking this.
hidden building Cost
Changes During Construction
One of the biggest causes of overspending is changing the design once building has started.
Changes may involve:
- Additional demolition
- New materials
- Reordering
- Wasted materials
- Extra labour
- Programme delays
FMB recommends agreeing the price and time consequences of a variation before the additional work proceeds.
Client Upgrades
Not every additional cost is genuinely unforeseen.
Sometimes the original budget grows because the client chooses:
- More expensive kitchen
- Better tiles
- Larger rooflights
- Premium flooring
- Bespoke joinery
- Additional lighting
These should be kept separate from genuine unexpected costs.
Builder Variations
A variation is a change to the agreed scope.
It may arise because:
- Client changes something
- Unforeseen work is discovered
- Drawings change
- Additional work becomes necessary
Good practice is to agree:
- What is changing
- Cost
- Programme impact
in writing before the work proceeds.
Delays
Delays can generate indirect costs.
For example:
- Longer scaffold hire
- Extended accommodation
- Storage
- Additional finance costs
- Contractor preliminaries
A one-month delay may therefore cost more than simply one extra month of labour.
Material Price Changes
Long construction programmes can be exposed to changing material prices.
Depending on the contract, price increases may be:
- Absorbed by contractor
- Passed to client
- Shared
The contract should make clear how this is handled.
Inflation Between Design and Construction
If the project is designed today but does not start for 12–18 months, today’s cost estimate may no longer be realistic.
Budget reviews should therefore be carried out as the project progresses.
Finance Costs
Borrowing can create additional costs such as:
- Interest
- Arrangement fees
- Valuation fees
These are outside the construction budget but still part of the homeowner’s actual financial commitment.
Contingency
Every building project should include contingency.
FMB recommends around 10% for many projects and suggests 15% or more for older homes or work involving structural alterations. FMB
A contingency is there for genuine unknowns such as:
- Hidden structural defects
- Drainage issues
- Ground conditions
- Old wiring
- - Rotten timber
It should not be treated as extra money available for upgrades.
hidden building Cost
£50,000 Building Quote: What Should You Budget?
If the construction quote is:
£50,000
you should not automatically assume £50,000 is the project budget.
You may still need:
- Design fees
- Structural engineer
- Planning
- Building Control
- Finishes
- VAT
- Contingency
A 10% contingency alone would add:
£5,000
and that is before any other excluded costs.
£100,000 Building Quote: What Should You Budget?
On a £100,000 project:
10% contingency = £10,000
15% contingency = £15,000
If architectural, engineering, statutory and finishing costs sit outside the builder’s figure, the total amount required can exceed £100,000 considerably.
The safest approach is to build a complete project budget before committing to construction.
Hidden Costs in Extensions
Common extension extras include:
- Structural engineer
- Building Control
- Drainage changes
- Party-wall surveyor
- Steelwork
- Scaffolding
- Kitchen
- Flooring
- Landscaping
Hidden Costs in Loft Conversions
Potential loft conversion extras include:
- Structural engineering
- Fire-safety upgrades
- New doors to existing rooms
- Staircase
- Party-wall matters
- Scaffolding
- Temporary roof
- Bathroom
- Roof repairs
Hidden Costs in Renovations
Renovations contain greater uncertainty because the existing building is being opened up.
Potential discoveries include:
- Damp
- Rotten timber
- Old wiring
- Leaking plumbing
- Structural defects
- Poor historic workmanship
How to Avoid Hidden Building Costs
You cannot eliminate every unknown.
But you can reduce surprises substantially.
Before construction:
- Develop the design properly
- Obtain technical drawings
- Coordinate structural engineering
- Investigate drainage where necessary
- Obtain comparable quotations
- Check exclusions
- Check allowances
- Check VAT
- Agree contract terms
- Build in contingency
The less information available when the builder prices the project, the more assumptions are likely to appear later.
Read Every Exclusion
One of the most important parts of a quotation is often the exclusion list.
Look for statements such as:
- Client supply
- By others
- Excluded
- To be confirmed
- Provisional
- Subject to investigation
These phrases identify where costs may still change.
Check the Allowances
An allowance is only useful if it is realistic.
For example, if you know you want a £20,000 kitchen but the builder has allowed £8,000, the quotation is already £12,000 below the specification you intend to buy.
Do not wait until construction to discover that difference.
Compare Quotes Like for Like
Builders should ideally price the same:
- Drawings
- Structural information
- Specification
- Scope of works
Otherwise, a cheaper quote may simply contain more exclusions.
Builders should ideally price the same:
Check the Contract Before Construction
The contract should help establish:
- Contract sum
- Scope
- Payment terms
- Programme
- Variations
- Delays
- - Responsibility for different works
Clear contractual information cannot prevent every unexpected cost, but it can reduce confusion about who is responsible for paying it.
Hidden Building Cost Checklist
Before committing to the project, check whether the budget includes:
- Architectural fees
- Structural engineer
- Planning application
- Lawful Development Certificate
- Building Control
- Party-wall surveyor
- Specialist surveys
- Drainage
- Ground conditions
- Specialist surveys
- Scaffolding
- Temporary roof
- Skip hire
- Waste disposal
- Demolition
- Temporary works
- Steel delivery and installation
- Crane hire
- Parking permits
- Utility alterations
- Heating upgrades
- Electrical upgrades
- Kitchen
- Bathrooms
- Flooring
- Decoration
- External works
- Landscaping
- Making good
- Temporary accommodation
- Storage
- Insurance
- VAT
- Contingency
If several of these are missing from the builder’s quotation, they do not disappear.
They simply sit elsewhere in the project budget.
How Tholos Can Help Reduce Cost Surprises
Hidden building costs are often not truly hidden.
They are simply identified too late.
At Tholos Architects, we can help develop the project with greater clarity around:
- Scope
- Drawings
- Structure
- Approvals
- Builder information
- Exclusions
- Likely additional costs
before construction starts.
The objective is not to promise that no unexpected costs will ever arise.
That would be unrealistic.
The objective is to reduce avoidable surprises and give you a more realistic picture of what the complete project is likely to involve.
Client experience
"
Lorem ipsum dolor sit amet, consectetur adipiscing elit. We had a vision for our home but no idea how to realise it. Tholos Interiors took complete ownership of the project — every detail was considered, every decision explained. The result exceeded everything we had imagined.
– Lawrence Ipsum, Homeowner · London –
Frequently Asked Questions
What hidden costs should I budget for when building an extension?
Commonly missed costs include architectural and structural-engineering fees, planning, Building Control, Party Wall matters, scaffolding, waste disposal, kitchen upgrades, flooring, decoration, VAT and contingency. FMB identifies these among the expenses most often overlooked by homeowners. FMB
Are architect fees included in a builder quote?
Usually not unless specifically stated.
Architectural services are normally contracted separately.
Is a structural engineer included in the builder’s price?
Not necessarily.
Some design-and-build arrangements may include engineering, but otherwise structural-engineering fees are commonly separate.
How much does Building Control cost?
Fees vary considerably. Current guidance puts a full-plans application for a single-storey extension at approximately £250–£550, with additional inspection charges potentially around £330–£400.
How much can Party Wall costs add?
Current 2026 guidance puts a Party Wall Award at around £1,000, while a typical extension involving one surveyor may cost approximately £1,200–£1,500. Where separate surveyors are appointed, the total can increase significantly
How much does scaffolding cost?
Current UK guidance gives around £20–£25 per m², with approximately £875 as a typical benchmark for a two-storey semi-detached house scaffold arrangement
How much does skip hire cost?
Possible reasons include:
- Excluded items
- Unrealistic allowances
- Variations
- Unforeseen conditions
- Upgraded specification
- Additional professional or statutory costs
Understanding which category the increase falls into is important.
Budget for the Whole Project, Not Just the Building Work
The biggest budgeting mistake is assuming:
Builder’s quote = project cost.
It often does not.
A realistic budget should consider:
Construction + professional fees + statutory costs + finishes + external works + VAT + contingency.
That gives a much clearer picture of how much money the project may actually require.
At Tholos Architects, we can help you understand the project beyond the headline construction price and identify many of these costs before work reaches site.
Disclaimer
This article is part of a series created by our dynamic marketing team for informational purposes only. While we strive to review all content thoroughly, occasional oversights may occur, even after publication. Therefore, we cannot accept liability for any interpretations or use of the information provided. For the most up-to-date information or to report any discrepancies, please contact us at hello@tholos-architects.com.
Start your project