If you are planning building works on your home, you might be surprised to learn that you’re legally required to appoint a “Principal Designer” and, confusingly, you may actually need two different ones depending on the stage and type of your project. 

Most homeowners have never even heard of this role, so if you are confused, you are not alone. 

Let’s break it down in plain English. 

WHAT IS A PRINCIPAL DESIGNER? 

In the construction world, the Principal Designer is the person (or company) in charge of making sure your project is safe and legally compliant – during the design stage. They take responsibility for health and safety risks during planning, and more recently, for ensuring the design meets building regulations’ requirements under the new Building Safety Act. 

There are now two different types of Principal Designer, and each has a separate set of legal duties: 

1. The CDM Principal Designer 

This role comes from the Construction (Design and Management) Regulations 2015, often shortened to CDM. 

Their main job is to manage health and safety during the pre-construction (design) phase. That includes: 

• Planning how health and safety will be handled during design and construction 
• Making sure all designers (e.g., architect, structural engineer) are sharing safety information 
• Identifying and reducing risks before building work begins 
• Ensuring the Principal Contractor (usually your builder) is given all necessary health and safety info 

If you don’t appoint a CDM Principal Designer in writing, then by law you, the homeowner (called “the Client”), take on this responsibility and all the legal liability that comes with it. 

This role is needed whenever more than one contractor (including sub-contractors under the guidance of a main contractor, e.g. one bricklayer hires one electrician to work on your job) will be involved which, for most home projects, is nearly always the case. 

2. The BSA Principal Designer 

This is a new role introduced by the Building Safety Act 2022, and it became mandatory for many domestic projects starting in October 2023. 

The BSA Principal Designer’s responsibility is to make sure your design complies with Building Regulations. That is everything from fire safety and structural integrity to energy efficiency, ventilation, accessibility, plumbing and sanitation, electricity, contaminations and more.

They must confirm that: 

• The design meets all the relevant “Approved Documents” (from Part A to Part T) 
• Each specialist designer involved (e.g., engineers, sustainability consultants) has done their part correctly 
• The full design is coordinated, buildable, and safe 
• A formal sign-off is submitted before building work starts 

This role is legally required for all projects that require building control approval, which again, is almost all extensions, conversions, or structural changes. 

So… Do I Really Need Two Principal Designers? 

Yes – technically.
The law treats these as two separate roles because they cover two different areas: 

• CDM Principal Designer → Health & Safety risks during design and construction 
• BSA Principal Designer → Legal compliance of the building design itself 

However, in most domestic projects, a competent architectural practice (like ours) can carry out both roles under a single appointment, saving you time, costs and confusion. 

What Happens If I Don’t Appoint One? 

If you fail to appoint a Principal Designer: 

• You become legally responsible for the role, even if you are not aware of the requirements
• You could face delays, fines, or enforcement notices if things go wrong or are not documented properly 
• Building Control might refuse to issue your Completion Certificate, which is required to sell your home in future
• Your builder may not be covered by insurance if the right procedures are not in place 

When Should I Appoint Them? 

As early as possible, ideally at the beginning of a project but certainly as soon as your planning drawings are approved.

Early appointment means your Principal Designer can: 

• Develop a coordinated design vision with safety and building regulations already in mind
• Advise on safety and compliance before detailed drawings are prepared 
• Coordinate with your structural engineer and other specialists 
• Identify and avoid risks that might later cause delays or rework 
• Prepare and submit the required documentation to Building Control 

How Much Does It Cost? 

Fees vary depending on the size and complexity of your project, but a combined CDM and BSA Principal Designer appointment typically costs: 

💷 £2,000 – £15,000, depending on project complexity
(This usually represents 0.5% – 2% of your overall construction budget) 

It’s not a small cost – but compared to the risk of fines, delays or rebuilds, it’s money well spent. And with the right team, it becomes part of a seamless, stress-free process. 

What Should I Look for in a Principal Designer? 

You should only appoint someone who is: 

• Properly qualified (RIBA architects, for example, are well-placed for this) 
• Experienced in residential projects 
• Willing to take on both roles, so there’s no confusion 
• Covered by professional indemnity insurance 
• Clear about their responsibilities in writing 

At Tholos Architects, we act as both CDM and BSA Principal Designer for most of our clients. We’ve been doing it since the start of the regulations and know exactly what’s required to keep your project on track and compliant. 

Principal Designer in summary

Still Confused? You don’t have to be.

We have helped dozens of homeowners who had no idea these roles even existed.
We will explain exactly what you need, when, and why. No jargon, no hard sell. 

📞 Book a free 30-minute consultation and we will walk you through your next steps with no obligation to proceed.