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How Much Contingency Should You Allow for a Building Project?
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Project Contingency intro
How Much Contingency Should You Allow for a Building Project?
A project contingency is money set aside for costs that cannot reasonably be confirmed before construction starts.
Even with:
- Detailed drawings
- Structural calculations
- A clear specification
- A fixed builder quotation
- A written contract
some uncertainty remains when working with an existing building or excavating ground that has not yet been opened.
For most domestic building projects, around 10% of the total project cost is a sensible starting point.
For older homes, extensive renovations or projects involving significant structural alteration, 15% or more may be appropriate. Current 2026 Federation of Master Builders guidance recommends around 10% generally and a higher allowance of 15% or more where older properties or structural alterations create greater uncertainty. FMB
A contingency is not an invitation to spend more.
It is financial protection against things you could not reasonably know before construction began.
At Tholos Architects, we encourage homeowners to include contingency within the project budget from the beginning rather than hoping there will be enough money left if something goes wrong.
Project Contingency Explained
What Is a Project Contingency?
A project contingency is a reserved amount of money that is not allocated to planned work.
It exists to deal with genuine uncertainty.
For example, when a wall is opened during renovation, you may discover:
- Rotten timber
- Damaged masonry
- Poor previous alterations
- Old wiring
- Leaking pipes
When foundations are excavated, you might discover:
- Poor ground
- Existing structures
- Unexpected drainage
- Different foundation conditions
These costs were not necessarily missing from the quotation.
They may simply have been impossible to establish before construction.
How Much Contingency Should I Allow?
A useful broad guide is:
| Suggested Starting Contingency | |
|---|---|---|
| Straightforward modern-property alteration | 5–10% | |
| Typical extension | Around 10% | |
| Loft conversion | Around 10% | |
| Garage conversion | Around 10% | |
| Major house renovation | 10–15% | |
| Older property renovation | 15%+ | |
| Extensive structural alterations | 15%+ | |
| Complex historic or unknown-condition property |
|
FMB’s current renovation guidance recommends at least 10%, increasing to 15% or more for older properties or structural alterations.
Its wider renovation checklist suggests a 10–20% contingency where uncertainty is greater
10% Project Contingency
For many relatively straightforward domestic projects, 10% is a useful starting point.
For example:
| Project Budget | 10% Contingency |
|---|---|
| £30,000 | £3,000 |
| £50,000 | £5,000 |
| £75,000 | £7,500 |
| £100,000 | £10,000 |
| £150,000 | £15,000 |
| £200,000 | £20,000 |
A £100,000 project should therefore not necessarily be approached with exactly £100,000 available.
A safer early budget might be:
£100,000 planned project + £10,000 contingency = £110,000 available budget
15% Project Contingency
A higher contingency can be sensible where the project involves more uncertainty.
For example:
| Project Budget | 15% Contingency |
|---|---|
| £30,000 | £4,500 |
| £50,000 | £7,500 |
| £75,000 | £11,250 |
| £100,000 | £15,000 |
| £150,000 | £22,500 |
| £200,000 | £30,000 |
This may be more appropriate for:
- Victorian housesVictorian houses
- Period properties
- Major renovation
- Structural alterations
- Projects involving substantial demolition
- Properties with known defects
FMB specifically recommends considering 15% or more for older properties or projects involving structural alterations.
20% Project Contingency
A 20% contingency is not required for every project.
But it may be worth considering where uncertainty is particularly high.
Examples could include:
- Very old property
- Long-neglected building
- Major structural repair
- Extensive strip-out
- Listed building
- Property with damp or movement
- Complex basement work
- Significant unknown drainage
- Project with very little existing information
Current FMB renovation guidance advises homeowners to consider contingency in the 10–20% range depending on the risks involved.
Contingency Is Not the Same as Hidden Costs
This distinction is extremely important.
A contingency should cover unexpected costs.
It should not cover costs that were always going to exist but were forgotten from the budget.
For example:
Planned Cost
You know you need a structural engineer.
That is not contingency.
Hidden or Missing Budget Item
You forgot to include the structural engineer.
That is not contingency either.
Genuine Contingency
The engineer discovers that an existing beam is inadequate once the structure is exposed and additional strengthening is required.
That may legitimately come from contingency.
Read more about Hidden Building Costs.
Hidden Building Costs
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What Should a Contingency Cover?
A project contingency may reasonably be used for:
- Unexpected structural defects
- Unforeseen drainage
- Poor ground conditions
- Rotten timber
- Hidden damp damage
- Unsafe existing wiring
- Unknown pipework
- Previously concealed defective work
- Additional foundation requirements
- Necessary remedial work
The common feature is that these were not reasonably predictable when the original budget was prepared.
What Should Contingency Not Cover?
Contingency should not normally be used for:
- More expensive kitchen
- Premium flooring
- Better tiles
- Larger rooflights
- Additional rooms
- Upgraded appliances
- Bespoke joinery
- Changes of mind
These are usually:
client changes or specification upgrades
rather than unforeseen project costs.
FMB specifically advises that a contingency fund should be reserved for genuine unexpected issues rather than optional upgrades.
Do Not Spend the Contingency Before Construction Starts
One of the easiest ways to lose the protection of a contingency is to treat it as spare budget.
For example:
Initial construction budget:
£80,000
Contingency:
£8,000
Total available:
£88,000
Then, before construction:
- £3,000 kitchen upgrade
- £2,000 better glazing
- £2,500 premium flooring
Only:
£500
of the original contingency remains.
The project has effectively started without meaningful contingency.
Project Contingency Explained
Construction Contingency vs Total Project Contingency
You also need to decide what figure the percentage is being applied to.
Suppose your project includes:
- Construction: £80,000
- Architect and engineer: £5,000
- Approvals: £1,500
- Kitchen: £15,000
Total planned project:
£101,500
A 10% contingency calculated only on the builder’s £80,000 quotation would be:
£8,000
A 10% contingency calculated on the entire £101,500 project would be:
£10,150
For early homeowner budgeting, applying contingency to the real project budget rather than only one contract figure often gives a more useful safety margin.
Builder Contingency vs Client Contingency
A builder may already have included some commercial risk within their price.
That is different from the homeowner’s contingency.
Your own contingency needs to protect the wider project.
It should therefore not be assumed that:
“The builder has allowed something for risk, so I don’t need contingency.”
The builder is protecting their business.
Your contingency protects your project budget.
Contingency and Fixed-price Contracts
A fixed-price building contract does not mean there can never be additional costs.
The contract may fix the cost of the agreed scope.
But additional cost can still arise from:
- Client variations
- Unforeseen conditions
- Work outside the agreed scope
- Changes required after opening existing construction
The contract should explain how these situations are dealt with.
Check a Builder Contract
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Contingency and Builder Quotes
A detailed builder quotation reduces uncertainty.
It should ideally make clear:
- Included work
- Excluded work
- Allowances
- Provisional sums
- VAT
- Assumptions
The more unclear the quotation is, the harder it becomes to distinguish between:
- Genuine unforeseen work
- Missing scope
- Builder variation
- Client upgrade
Provisional Sums Are Not Contingency
A provisional sum is an allowance within the quotation for work that cannot yet be priced precisely.
For example:
Drainage provisional sum: £3,000
That £3,000 is already part of the project budget.
Your contingency sits outside it.
If the final drainage work costs £4,500, the extra £1,500 may need to come from contingency.
Prime Cost Allowances Are Not Contingency
The same principle applies to product allowances.
For example:
Bathroom sanitaryware allowance: £4,000
If you deliberately choose products costing £6,000, the additional £2,000 is not really an unforeseen contingency cost.
You upgraded the specification.
Project Contingency for an Extension
A straightforward extension should generally have contingency available for issues such as:
- Ground conditions
- Existing foundations
- Drainage
- Structural discoveries
- Existing services
Where the extension significantly alters an older property, a higher percentage may be appropriate.
FMB currently recommends 10% generally and 15% or more where structural work or older buildings increase risk.
Single-storey Extension Contingency
A relatively straightforward single-storey extension may have fewer unknowns than a major whole-house renovation.
However, uncertainty can still arise from:
- Excavation
- Drainage
- Existing walls
- Service routes
A 10% contingency remains a sensible starting point.
Single-storey Extension Cost
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Wraparound Extension Contingency
Wraparound extensions can carry more risk because they often involve:
- Rear and side foundations
- Existing drainage
- Multiple structural openings
- Larger amounts of existing-house alteration
A contingency toward 10–15% may therefore be more comfortable depending on the age and condition of the property.
Loft Conversion Contingency
A loft conversion involves opening an existing roof and modifying existing structure.
Possible unknowns include:
- Existing timber condition
- Previous roof alterations
- Chimney structure
- Party-wall conditions
- Roof covering condition
Around 10% is a useful starting point for many loft conversions.
More complicated older properties may warrant additional allowance.
Garage Conversion Contingency
A straightforward modern garage conversion may carry relatively limited uncertainty.
However, unexpected costs can still arise from:
- Damp
- Poor existing floor
- Insufficient insulation
- Roof defects
- Existing services
Around 10% can still be a useful starting point.
House Renovation Contingency
Renovations generally require more contingency than straightforward new construction because you are working with an existing building.
Current 2026 guidance from Checkatrade recommends 10–15% for unexpected house-renovation costs, while FMB suggests higher allowances for older houses and structural work.
Possible discoveries include:
- Damp
- Rot
- Old plumbing
- Unsafe wiring
- Structural defects
- Failed previous repairs
Older House Contingency
Older properties generally contain greater uncertainty.
A Victorian house may contain:
- Numerous historic alterations
- Old wiring
- Old drainage
- Solid-wall construction
- Timber decay
- Historic damp treatments
Some may have been altered several times by previous owners.
This is why 15% or more may be more sensible than trying to operate with the absolute minimum buffer.
Project Contingency Explained
Listed Building Contingency
Listed-building work can introduce additional uncertainty because:
- Existing historic fabric needs greater care
- Specialist materials may be required
- Repair may be preferable to replacement
- Additional consent may be needed if discoveries alter the proposed work
A larger contingency can therefore be sensible.
Structural Work Contingency
Projects involving substantial structural alterations generally carry greater risk.
This includes:
- Removing several load-bearing walls
- Installing structural frames
- Altering existing roofs
- Repairing existing structure
FMB specifically identifies structural alteration as a reason to increase contingency beyond the normal 10% starting point
Foundation Contingency
Before excavation, foundation cost is based partly on assumptions about the ground.
Unexpected conditions may include:
- Multiple levels
- Soft soil
- Tree roots
- Old foundations
- Buried structures
The structural solution may then need to change.
That can affect:
- Excavation
- Concrete
- Labour
- Engineering
Drainage Contingency
Drainage is another common area of uncertainty.
Drawings and surveys can reduce the risk, but existing drainage may still differ from assumptions.
Potential issues include:
- Unexpected drain routes
- Damaged pipes
- Hidden chambers
- Shared drainage
- Incorrect historic alterations
Some contingency should remain available until the drainage work has been confirmed.
Electrical Contingency
Opening an older building may reveal:
- Unsafe wiring
- Undersized circuits
- Old consumer unit
- Poor DIY alterations
The scope can then increase from:
“Move a few sockets”
to:
“Upgrade the existing installation.”
That is exactly the type of existing-building uncertainty contingency can help absorb.
.
Plumbing Contingency
Similarly, existing plumbing may reveal:
- Corroded pipework
- Poor pressure
- Leaks
- Old waste systems
- Inadequate hot-water capacity
Additional work may become necessary once the system is altered.
Roof Contingency
Roof work can uncover:
- Rotten rafters
- Damaged battens
- Failed membrane
- Inadequate historic repairs
- Chimney defects
A loft conversion or renovation may therefore justify a larger roof-related allowance than a completely new extension.
Contingency and Inflation
A contingency is primarily intended for project uncertainty.
It should not automatically be relied upon to cover general inflation.
If there is a long gap between:
- Cost estimate
- Builder quote
- Construction start
the project budget itself should be updated.
Contingency and Material Price Changes
Current FMB guidance notes that changing labour and material costs remain a consideration for homeowners planning works in 2026.
Whether price changes affect the homeowner depends partly on:
- Contract
- Procurement
- When materials are ordered
- Whether the price is fixed
This should be distinguished from genuine unforeseen construction work.
Project Contingency
Contingency and Client Changes
Client changes are one of the easiest ways to exceed a budget.
Examples include:
- Moving windows
- Changing layout
- Upgrading kitchen
- Adding rooflights
- Moving bathroom
FMB recommends agreeing the price and programme effect of changes before the additional work proceeds.
These costs should ideally be tracked separately from contingency.
Keep a Variation Log
During construction, keep a simple record showing:
| Item | Reason | Cost | Contingency? |
|---|---|---|---|
| Drain repair | Unexpected damage | £1,200 | Yes |
| Kitchen upgrade | Client choice | £3,000 | No |
| Extra steel | Existing structure discovered | £1,500 | Potentially |
| Premium tiles | Client choice | £800 | No |
This helps prevent every additional cost being described as an “unexpected extra”.
Do Not Hide Overspending Inside Contingency
It is useful to separate project costs into:
Original budget
Approved client upgrades
Unforeseen contingency expenditure
That allows you to understand why the final budget changed.
Without this separation, it is very easy to say:
“The project went £15,000 over budget.”
when £10,000 of that amount was actually additional work chosen during construction.
When Should Contingency Be Released?
Do not assume the contingency is available for spending simply because construction is halfway through.
Risk reduces gradually.
For example:
Once foundations are complete, much of the ground-condition risk has passed.
Once the existing structure has been opened, structural uncertainty reduces.
Once first fix is complete, services become clearer.
The contingency can therefore be reviewed progressively rather than released early.
Keep Contingency Until the End
Ideally, some contingency should remain until:
- Construction is substantially complete
- Major defects have been resolved
- Final variations are known
- Final account is understood
Spending the last contingency money on upgrades before completion removes your protection at exactly the point when final costs are becoming clear.
Cash Contingency vs Borrowing Capacity
A contingency works best when the money is actually accessible.
A theoretical £15,000 contingency is not particularly useful if there is no realistic way to fund it.
FMB’s self-build guidance recommends considering not only the contingency percentage but also what options would be available if more funds were unexpectedly required.
£50,000 Project Contingency Example
Suppose the planned project costs:
£50,000
A 10% contingency would be:
£5,000
Total available project budget:
£55,000
For an older property at 15%:
£7,500 contingency
Total available:
£57,500
£75,000 Project Contingency Example
Planned project:
£75,000
At 10%:
£7,500 contingency
Total:
£82,500
At 15%:
£11,250 contingency
Total:
£86,250
£100,000 Project Contingency Example
Planned project:
£100,000
At 10%:
£10,000
Total:
£110,000
At 15%:
£15,000
Total:
£115,000
At 20%:
£20,000
Total:
£120,000
£150,000 Project Contingency Example
Planned project:
£150,000
At 10%:
£15,000
Total:
£165,000
At 15%:
£22,500
Total:
£172,500
At 20%:
£30,000
Total:
£180,000
Project Contingency
What If I Cannot Afford the Contingency?
If the project uses every pound of available funding before construction starts, the budget is vulnerable.
For example:
Available money:
£100,000
Design currently costs:
£100,000
Rather than saying:
“Hopefully nothing goes wrong,”
a safer response may be to reduce the planned scope or specification so that contingency remains.
FMB similarly advises that if maintaining an adequate contingency requires reducing some elements of the project, scaling back can be preferable to beginning with no financial buffer.
Design to the Budget Minus Contingency
A useful way to think about this is:
If you have:
£100,000 total available
and want:
10% contingency
you should not necessarily design a £100,000 project.
Your planned construction and associated costs may need to sit closer to:
£90,000–£91,000
so there is still a meaningful reserve.
Better Information Can Reduce Risks
Contingency is not a substitute for proper design.
Uncertainty can often be reduced through:
- Measured survey
- Detailed architectural drawings
- Structural engineering
- Drainage investigations
- Technical information
- Clear specification
Better information can reduce the number of assumptions made during pricing.
Structural Design Can Reduce Cost Uncertainty
Structural information developed before builder pricing can help identify:
- Beam sizes
- Columns
- Foundations
- Structural openings
That makes it easier for contractors to price the actual work rather than make provisional assumptions.
Detailed Builder Quotes Reduce Uncertainty
A builder cannot accurately price information they do not have.
The quotation becomes more useful when it is based on:
- Defined drawings
- Structural design
- Specification
- Agreed scope
FMB recommends breaking the budget into clear categories and asking builders to explain the assumptions made in their quotations.
A Contingency Does Not Replace a Good Contract
Having £10,000 available does not mean every request for another £2,000 should simply be accepted.
When additional costs arise, establish:
- Why is the work required?
- Was it included originally?
- Could it reasonably have been foreseen?
- Who requested the change?
- What does the contract say?
- What is the price?
- Does it affect the programme?
Contingency protects the budget.
The contract helps establish responsibility.
Project Contingency
Project Contingency Checklist
Before construction starts, ask:
- What is the total planned project cost?
- Does it include VAT?
- Are professional fees included?
- Are statutory fees included?
- Are kitchens and bathrooms realistic?
- Are provisional sums identified?
- What existing-building risks remain?
- What ground risks remain?
- Is the property old or heavily altered?
- Is significant structural work involved?
- IHave I allowed at least 10% contingency?
- Should that be closer to 15–20%?
- Is the contingency money actually available?
- Have I kept upgrades separate from contingency?
If your budget only works when absolutely nothing unexpected happens, it is probably too tight.
That principle is echoed in current FMB guidance, which recommends retaining a meaningful contingency rather than budgeting only for the best-case scenario.
How Tholos Can Help Reduce Project Risk
The purpose of contingency is not to compensate for poor planning.
A well-developed project should try to identify as many costs as reasonably possible before construction.
At Tholos Architects, we can help reduce uncertainty through:
- Design development
- Architectural drawings
- Technical information
- Structural coordination
- Planning
- Building Regulations coordination
- Clearer information for builder pricing
There will always be some uncertainty when working with existing buildings.
The objective is to make the unknown part smaller before construction begins.
Then the contingency remains what it should be:
protection against genuine surprises, rather than money needed to complete things that should have been budgeted from the beginning.
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 percentage contingency should I allow for a building project?
Around 10% is a sensible starting point for many domestic projects. FMB recommends considering 15% or more for older properties or projects involving structural alterations.
Is 10% contingency enough?
For a relatively straightforward, well-defined project, it may be.
Projects involving older properties, significant structural alterations or more uncertainty may justify 15–20%.
What is a 10% contingency on £50,000?
£5,000.
That means the total funds available would ideally be around £55,000.
Should contingency include VAT?
When setting a total homeowner budget, calculate from realistic amounts you actually expect to pay.
If VAT is payable, budgeting from VAT-exclusive figures can understate the real amount required.
Is contingency included in a builder quote?
Usually not as the homeowner’s own contingency.
Some quotations contain provisional sums or contractor allowances, but these are different from the client’s reserve fund.
Can I use contingency for a better kitchen?
You can spend your own money however you choose, but a kitchen upgrade is not genuinely an unforeseen contingency item.
Using contingency for upgrades reduces the protection available if genuine unexpected costs arise later.
What happens if I don’t use the contingency?
Excellent.
It remains your money.
A contingency is protection, not an amount that needs to be spent.
Can I use contingency for a better kitchen?
A Good Contingency Is Money You Hope You Never Need
For most domestic building projects, around 10% is a sensible initial contingency.
For older homes, renovations and substantial structural alterations, 15% or more may provide a more realistic safety margin. Current FMB guidance supports both of these benchmarks and, for renovation projects with greater uncertainty, suggests considering up to around 20%.
The important thing is to separate:
planned costs + known additional costs + contingency
rather than expecting one figure to cover everything.
A project that completes without using its contingency has not been over-budgeted.
It has been properly protected against risk.
At Tholos Architects, we can help identify more of the project’s likely costs before construction begins, allowing the contingency to remain reserved for the things that genuinely could not have been known.
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.
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