What is a Principal Designer, and do I need one for my home extension?

You’ve had a quote back from your builder, or maybe your solicitor mentioned it in passing, and now you’re staring at the words “principal designer” wondering what on earth they mean. It sounds grand. It sounds expensive. And you’re not sure whether it’s a legal requirement or just another line on someone’s invoice. The good news is it’s simpler than it sounds, and by the end of this page you’ll know exactly where you stand.

There are actually two different Principal Designer roles.

This is where most explanations get it wrong. They talk about “the principal designer” as if it’s a single thing. It isn’t. Since 2022, there are two separate roles with different legal duties, and they come from two completely different pieces of legislation.

The first is the CDM Principal Designer (CDM-PD). CDM stands for the Construction (Design and Management) Regulations 2015, and this role is about keeping people safe during the construction process itself: the builders, the trades, anyone on site while the work is happening. It’s been around for years.

The second is the Building Regulations Principal Designer (BR-PD). This one is newer, introduced under the Building Safety Act 2022, and it’s focused on a different question entirely: will the finished building be safe and compliant for the people who live in it? Structure, fire safety, thermal performance, drainage, everything the building regulations exist to protect. Different law, different purpose, different set of duties.

They’re often appointed to the same person, typically your designer or architectural technologist, but they are legally separate appointments, and each carries its own responsibilities.

What does the CDM Principal Designer do?

When designers draw up plans for your extension, they’re making decisions that affect how safely it can be built. The depth of your foundations, the size of the steelwork, how the roof connects to the existing structure. All of these have safety implications on site.

The CDM Principal Designer plans and manages health and safety during the design phase. They make sure every designer involved in the project, your structural engineer, your drainage consultant, anyone contributing to the design, has properly considered how their work will be built safely. They coordinate between these designers so that one person’s solution doesn’t create a hazard for another trade on site. The duties aren’t vague: the industry guidance for principal designers sets them out in practical detail.

You’re legally required to appoint one whenever your project involves more than one contractor. In practice, that threshold is lower than most people expect.

What does the Building Regulations Principal Designer do?

Where CDM is about safety during construction, this role is about safety and performance during the building’s use and occupation. The building regulations set minimum standards for how a finished building must perform, structurally, thermally, in a fire, for drainage, ventilation, accessibility, and the Building Regulations Principal Designer coordinates the design to make sure it meets all of them before your builder breaks ground. In practice, that means ensuring the building regulations drawings and specifications are compliant as a complete package, not just checking individual elements in isolation.

It’s a newer role, and many builders and even some designers aren’t fully across it yet. But the duty is clear: someone needs to take responsibility for confirming that the design, taken as a whole, will produce a building that’s safe and compliant for the people living in it. Without that sign-off, your building control surveyor cannot issue a completion certificate at the end of the project.

That completion certificate matters. Without it, you can face problems when you come to sell, remortgage, or insure the property. The Building Regulations Principal Designer exists to make sure you get there.

When do I need to appoint a Principal Designer?

For almost all residential construction projects, you need both.

If your project requires building regulations approval, and that covers structural alterations, extensions, loft conversions, most remodelling work, you need a Building Regulations Principal Designer.

If your project involves more than one contractor, you need a CDM Principal Designer. And the moment a specialist trades contractor is working alongside your main builder, an electrician, a structural steel erector, a window installer, you have more than one contractor. Unless a single builder can carry out every element of the work themselves, CDM applies. For extensions and loft conversions, that means virtually every project.

The only scenario where either role might genuinely not apply is very minor internal work with no structural element and a single contractor doing everything. If you’re reading this page, your project almost certainly isn’t that.

What happens if I don’t appoint one?

The duties don’t disappear. If you don’t appoint a Principal Designer, the legal responsibilities fall back on you as the client. Most homeowners have no idea they’re carrying that liability, and they wouldn’t know where to start in meeting those duties.

This isn’t about being alarmist. It’s about understanding that these roles exist for a reason, and that someone on your project needs to be doing the work. Appointing the right person means that work is done properly, documented clearly, and doesn’t land on your shoulders.

Who can be appointed as Principal Designer?

Not just anyone. The person you appoint needs to be competent: relevant qualifications, working knowledge of the building regulations and CDM, and enough experience to coordinate a design team and identify risks before they reach site.

In practice, this is usually an architect or local Sussex architectural technologist, provided they have the right knowledge and experience. It makes sense for the person drawing your plans to hold these roles, because they’re already making the design decisions that both roles are there to coordinate. Splitting the appointment away from the designer creates gaps. Keeping it together means compliance is built into the design from day one, not bolted on afterwards.

This isn’t a box-ticking exercise. A good Principal Designer genuinely reduces risk, on site, in the finished building, and for you as the client.

When should each Principal Designer be appointed?

The two roles have different appointment timelines, and getting the timing right matters.

The CDM Principal Designer should be appointed as early as possible, ideally before any design work begins. Their job is to coordinate health and safety thinking from the very start of the design process, so appointing them late means decisions have already been made without their input.

The Building Regulations Principal Designer is typically appointed at RIBA Stage 3 or Stage 4, once the design is developed enough for building regulations compliance to be coordinated in detail. This is when the technical design work is being pulled together and the building control application is being prepared.

In practice, both roles are often held by the same person, but the formal appointments happen at different points in the project. Your designer should be able to explain when each appointment needs to be made and what it covers.

Not sure what your project needs?

If you’re planning an extension, loft conversion, or new build in Sussex or Surrey and you’re not sure whether you need a Principal Designer, or who should be appointed, get in touch. I’m happy to talk it through before you commit to anything.