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If you have built before, you might remember the “Principal Designer” as the person who managed health and safety. Under the new Building Safety Act 2022, the title remains the same, but the job has changed completely.

There are now effectively two Principal Designers to be aware of:

1. CDM Principal Designer (The Old Role): Focuses on Health & Safety during construction.
2. Building Regulations Principal Designer (The New Role): Focuses on Design Compliance (ensuring the finished building meets all Building Regulations).

Note: This guide focuses on the new Building Regulations Principal Designer (PD).

Why the Client Needs One

The days of “build it now, fix the paperwork later” are over. Under the Building Safety Act, a Principal Designer must be in place before construction begins.

• For Commercial Clients: You have a strict legal duty to appoint a Principal Designer in writing. If you fail to do so, the legal duties (and liabilities) default back to you, the client.
• For Homeowners (Domestic Clients): The regulations are designed to protect you. While you should appoint a Principal Designer in writing, if you overlook this, the role automatically defaults to your lead designer (usually the Architect).
• Why You Should Still Appoint in Writing: Relying on an “automatic default” can lead to confusion and disputes about who is responsible for what. It is far safer to agree this explicitly in your contract with the architect or a specialist firm.

Why Structural Engineers Usually Do Not Act as Principal Designers

Clients often ask structural engineers to take on this role. While engineers are experts in Part A (Structure), they are typically not the right choice for the PD role.

• Scope of Competence: To be a Principal Designer, one must be competent to judge the compliance of the entire building. A structural engineer is not trained to assess fire escape distances or thermal insulation values. They cannot legally sign off on work outside their competence.
• Control of the Design: The regulations state the PD should be the designer “in control of the design phase.” On most projects, the structural engineer is a consultant supporting the design, not leading it. They don’t decide where the walls go; they just make sure they stand up.
• Insurance: Because the role carries liability for the whole building’s compliance, standard Structural Engineering Professional Indemnity (PI) insurance often excludes this role.

Who Should Take the Role?

1. The Architect / Lead Designer: For most domestic projects, this is the most natural fit as they have the broadest overview of the Building Regulations (fire, layout, insulation, etc.).
2. Specialist Building Safety Companies: If your project is complex, or your architect declines the role (which is becoming common due to insurance costs), there are now specialist companies and consultancies dedicated entirely to fulfilling the Principal Designer role. They can be hired independently to oversee compliance.

The Bottom Line: Ensure a Principal Designer is appointed early. If your architect or engineer declines the role, you must find a specialist third party who accepts it.