What Is a Principal Designer? The Essential Guide for Homeowners Planning an Extension or Renovation

If you’re planning a home extension, loft conversion, kitchen renovation, or even building a new house, you’ve probably spent plenty of time thinking about design ideas, finishes and budget.

But there’s one role most homeowners have never heard of — even though it’s now one of the most important legal requirements on any domestic building project:

⭐ The Principal Designer

And no, this isn’t the person choosing your tiles or colour palette.

The Principal Designer is the professional legally responsible for planning, managing and coordinating the design so your project is:

• safe to build

• compliant with Building Regulations

• properly coordinated between all designers

• documented correctly for Building Control

• designed to avoid expensive on-site mistakes

Since October 2023, almost every home renovation or extension in England requires one.

Let’s break down what that means in plain English.

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Why do I need a Principal Designer for my home project?

For years, homeowners often built from partial drawings or uncoordinated inputs from different consultants. This commonly led to:

• unexpected structural problems

• drainage issues

• errors on site

• cost overruns

• delays

• Building Control challenges

• safety concerns

The Government strengthened the rules under the Building Safety Act to ensure every project — even domestic extensions — has someone responsible for managing design and safety from the start.

That person is the Principal Designer.

What this means for you as a homeowner

Having a Principal Designer in place helps to:

• Reduce stress by keeping responsibilities clear and coordinated from the outset

• Protect your budget by identifying risks early, before they turn into costly changes

• Avoid delays and redesigns by resolving issues at the right stage, not on site

• Give peace of mind that your project is being checked, managed, and guided properly

What does a Principal Designer actually do?

Most homeowners assume their builder or architect “handles everything,” but the law now makes it clear:

“Someone must be responsible for coordinating design work and ensuring it complies with Building Regulations.”

That “someone” is the Principal Designer.

Here’s what they do — in simple terms.

1. They coordinate the entire design team

Even a small home extension usually involves:

• an architect

• a structural engineer

• occasionally a drainage or energy consultant

• a contractor providing input on buildability

The Principal Designer ensures:

• everyone is working from the same information

• designs don’t contradict each other

• structural and architectural layouts align

• drainage, ventilation and energy requirements are integrated

• all design changes are recorded and controlled

This coordination is where many domestic projects go wrong — and where the Principal Designer provides huge value.

2. They ensure the design meets ALL Building Regulations

Building Regulations govern:

• structure

• fire safety

• thermal performance

• ventilation

• energy efficiency

• drainage

• access and guarding

• overheating

• sound insulation

• glazing safety

The Principal Designer checks the design is:

• complete

• coordinated

• compliant

• ready for Building Control approval

This dramatically reduces the risk of delays, redesigns, and compliance problems on site.

3. They confirm every designer is competent

The Principal Designer must check that:

• each designer has the right skills, knowledge and experience

• no one is designing elements outside their ability

• contractors aren’t making unofficial design changes

This protects you from unsafe or non-compliant work.

4. They keep the project’s “Golden Thread” of information

The “Golden Thread” is a clear, traceable record of all key design information and decisions.

The Principal Designer:

• manages the official drawing set

• ensures version control

• coordinates documents between designers

• tracks updates and amendments

• prepares information for Building Control submission

This keeps your project organised, auditable, and compliant — especially important if you sell your home later.

5. They make sure the project is safe to build

Under the CDM Regulations, the Principal Designer must:

• identify risks early

• reduce or eliminate foreseeable hazards

• communicate these to the contractor

• ensure a Construction Phase Plan is prepared

This helps prevent:

• unsafe structural openings

• inappropriate temporary works

• working-at-height hazards

• risks to you or your family during construction

6. They prepare the Fire Safety Information at completion

For any extension or renovation, the Principal Designer ensures you receive:

• fire-stopping details

• escape route information

• fire strategy assumptions

• essential safety features for future maintenance

This is required under Regulation 38 for Building Control sign-off.

7. They sign the Principal Designer Completion Statement

At the end of the project, the Principal Designer must legally confirm they took “all reasonable steps” to coordinate the design and ensure compliance.

Without this, Building Control may refuse to issue a completion certificate.

What happens if you start work without a Principal Designer?

It’s one of the most common oversights in home projects, simply because the role isn’t widely understood. Without a Principal Designer in place from the start, small gaps in coordination can quickly turn into bigger issues on site.

Many homeowners begin work thinking:

“My builder will sort it.”

Unfortunately, this leads to significant risks:

❌ Building Regulations non-compliance

❌ Unsafe design changes on site

❌ Structural or drainage errors

❌ Disputes between builder and homeowner

❌ Costly rework and delays

❌ Completion certificate refusal

And importantly:

⭐ When a Principal Designer isn’t appointed, those duties can fall to the homeowner. Having the right support in place helps keep responsibilities clear and stress-free.

Most homeowners understandably don’t want that responsibility.

How the Principal Designer protects your project

A good Principal Designer helps you:

• avoid expensive mistakes

• reduce on-site risks

• keep the design coordinated

• meet Building Regulations

• get Building Control approval the first time

• keep the project running smoothly

They protect your home, your budget, and your peace of mind.

⭐ Want to know how the Principal Designer fits into the wider architectural process? You can read about the full journey from the first ideas through to completion in “Our Process”

Check Our Process

The Takeaway for Homeowners

A Principal Designer may sound like an extra layer of formality, but for most homeowners it’s the role that keeps a project safe, coordinated and free from avoidable surprises. By managing compliance, aligning every designer and maintaining the “Golden Thread” of information, they help prevent the costly mistakes and on-site confusion that often cause stress. 

With this support in place, your renovation feels clearer and more controlled from day one. If you’d like help understanding how this applies to your own extension or improvement, we’re here to guide you step by step with clarity and confidence.

If you’re planning a renovation, extension or new build and want to make sure your project is safe, compliant and properly coordinated, I’d be happy to guide you through the process.

Whether you need:

• help understanding your responsibilities

• support appointing the right dutyholders

• a coordinated Building Regulations package

• or full architectural services

…I can make the process clear and stress-free.

👉 Get in touch today to discuss your project.

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Frequently Asked Questions (FAQs)

1. Do I really need a Principal Designer for a small extension?

Yes — if more than one contractor is involved (which is almost always the case), the law requires one.

2. Isn’t my builder the Principal Designer?

Usually no. Builders rarely have the competence to coordinate structural, architectural, drainage and regulatory design requirements.

3. Is the Principal Designer the same as my architect?

Often yes — but only if you formally appoint them and they accept the role. An architect is not automatically the Principal Designer.

4. What happens if I don’t appoint one?

You become the Principal Designer by default, taking on all legal duties and liabilities.

5. Does the Principal Designer supervise the builder?

No. They plan, manage and coordinate the design, not the day-to-day construction work.

6. When should the Principal Designer get involved?

As early as possible — ideally before technical design begins and before any work starts on site.