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Principal Designer vs Principal Contractor Under the BSA — What's the Difference?

Principal Designer (PD) coordinates design phase under CDM 2015 + Building Regulations Part 2A (post-BSA 2022); manages design risk + ensures Building Regs compliance design. Principal Contractor (PC) manages construction phase; coordinates contractors + supervises work safety + Building Regs compliance during build. Both must demonstrate competence. Mandatory dutyholders on all building work since October 2023. Domestic Client transfers duties to PC if appointed alone.

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Principal Designer (PD) — role + scope

PD under CDM 2015 (existing) + Building Regulations Part 2A (BSA 2022 amendment from October 2023). Pre-existing CDM PD: coordinates pre-construction phase, manages design risk, ensures relevant info passed to PC. New BSA PD: additional duty to ensure design + plans comply with Building Regulations. Must be appointed before construction starts in writing. Competence: design qualifications + Building Regs knowledge + relevant experience. On Builderr projects: typically the architect appointed as PD if design-led; for design-and-build, Builderr's in-house project manager is PD. PD appointment includes formal acceptance + scope letter + competence statement.

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Principal Contractor (PC) — role + scope

PC under CDM 2015 + BSA 2022. Construction phase coordinator + competence verifier for trade contractors + supervises Building Regs compliance during construction + manages golden thread updates during build. Must be appointed before construction starts in writing. Competence: contractor qualifications (Trustmark, CIOB, FMB) + Building Regs experience + safety record + adequate insurance. Builderr is PC on all our build projects. PC duty includes: written method statements + competence checks on subcontractors + photographic record of compartmentation + fire-stopping + golden thread file.

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Domestic Client + dutyholder transfer

Where Client is a private individual having work on own home (Domestic Client): full statutory duties transfer to PC by default (or PD if no PC appointed). Domestic Client still has duty to appoint competent PD + PC + check competence + provide site access + cooperate. Practical impact for Builderr clients: (1) we explain duties at quote stage; (2) we provide written appointment letter + competence statement when accepting work; (3) we manage all statutory record-keeping + golden thread; (4) client receives golden thread file at handover. No additional cost to client — included in standard project management.

More questions

Related questions answered.

Can the architect be both PD and PC?

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Possible but rare. PD + PC roles have different competence requirements (design vs construction). Most projects: architect is PD, main contractor is PC. Some design-and-build contractors hold both roles (must demonstrate both competences).

What competence evidence is needed?

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Qualifications (degree, professional membership — RIBA, CIOB, FMB, Trustmark), training records, project portfolio, insurance certificates, references. PD specifically: Building Regs knowledge + CDM training (NEBOSH, IOSH). PC specifically: SMSTS, CSCS, Trustmark/FMB membership.

What if my contractor wasn't appointed in writing?

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Statutory breach — Client (domestic transfers to PC) liable. Practical remedy: agree backdated written appointment immediately. Without written appointment, no clear dutyholder = enforcement action by Building Control + potential insurance refusal.

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