Who is the Principal Contractor on your job?

If you've only ever worked with one other trade on a job, you might never have thought about who the Principal Contractor is. Then one day there are three trades on site at once and someone asks "who's PC on this?" — and it matters, because the answer decides who's legally responsible for coordinating everyone's safety on that site.

When the duty actually arises

The Construction (Design and Management) Regulations 2015 (CDM 2015) apply to all construction work in Great Britain, commercial or domestic, big or small. The duty to appoint arises under Regulation 5(1), which says that where there is more than one contractor, or if it is reasonably foreseeable that more than one contractor will be working on a project at any time, the client must appoint in writing a designer with control over the pre-construction phase as Principal Designer, and a contractor as Principal Contractor.

Two things in that sentence do a lot of work. The first is "reasonably foreseeable" — you don't wait until the second trade turns up. If you know at the outset that a sparks is coming in week three, the duty is already live. The second is "in writing". A conversation on site is not an appointment.

Regulation 5(2) requires those appointments to be made as soon as is practicable and, in any event, before the construction phase begins.

If you are genuinely working alone, with no other contractor involved at any point, those two roles don't come into play — but your own duties as a contractor under CDM 2015 still do, and so does your duty to assess the risks of your work under Regulation 3 of the Management of Health and Safety at Work Regulations 1999.

What happens if nobody is appointed

The duties don't disappear. On a commercial job, CDM 2015 is explicit about where they land:

• Regulation 5(3) — if the client fails to appoint a Principal Designer, the client must fulfil the Principal Designer's duties under Regulations 11 and 12.
• Regulation 5(4) — if the client fails to appoint a Principal Contractor, the client must fulfil the Principal Contractor's duties under Regulations 12 to 14.

So a client who "didn't get round to it" has not avoided anything. They have quietly appointed themselves.

What the Principal Contractor actually does on a small job

Strip away the paperwork and the job under Regulation 13 is to plan, manage, monitor and coordinate the construction phase so that work is carried out without risk to health and safety, so far as is reasonably practicable.

In practice, on a small job, that means making sure a construction phase plan exists and stays current (Regulation 12), organising how the trades work around each other without putting each other at risk, giving every worker a suitable site induction before they start, keeping unauthorised people off site, and consulting workers rather than simply instructing them.

None of that requires a big company. A sole trader running a two-trade domestic extension can be the Principal Contractor. The duties scale to the size of the job — they don't disappear because the job is small.

What the Principal Designer actually does

The Principal Designer's duties sit in the pre-construction phase under Regulation 11: planning, managing, monitoring and coordinating health and safety during design, helping the client assemble the pre-construction information, and passing it to whoever becomes Principal Contractor so the construction phase plan is built on real information rather than guesswork.

Regulation 12 also puts the health and safety file in the Principal Designer's hands — the document that follows the building afterwards, so that anyone maintaining or altering it later knows what is in the walls and how it was built.

The F10 notification — the exact threshold

This one gets misremembered constantly, so here it is as Regulation 6(1) actually words it. A project is notifiable if the construction work is scheduled to:

(a) last longer than 30 working days and have more than 20 workers working simultaneously at any point in the project; or (b) exceed 500 person days.

Read that carefully. Limb (a) needs both halves — long enough and busy enough. Limb (b) stands on its own. Either limb triggers notification.

Where a project is notifiable, Regulation 6(2) puts the duty to notify on the client, in writing, to the HSE, as soon as is practicable before the construction phase begins. In practice the Principal Contractor very often fills the F10 in and sends it on the client's behalf — but the duty sits with the client. Regulation 6(3) also requires the notice to be clearly displayed in the site office where workers can read it, and updated if necessary.

If your job meets neither limb, you don't need an F10. You still need everything else CDM requires.

Domestic clients — where this really catches people out

This is the part that surprises small contractors most, and it is worth getting exactly right.

Where the client is a domestic client — someone having work done on their own home, not in connection with a business — Regulation 7(1) moves the client duties in Regulations 4(1) to (7) and Regulation 6 onto:

• the contractor, where there is only one contractor;
• the Principal Contractor, where there is more than one contractor; or
• the Principal Designer, but only where there is a written agreement that the Principal Designer will take them on.

Then comes the provision that matters most. Regulation 7(2) says that if a domestic client fails to make the appointments required by Regulation 5, then the designer in control of the pre-construction phase is the Principal Designer, and the contractor in control of the construction phase is the Principal Contractor. Not "should be appointed as" — is.

And Regulation 7(3) confirms the consequence: Regulations 5(3) and 5(4) — the provisions that bounce unappointed duties back onto a commercial client — do not apply to a domestic client.

Put those together and the position on domestic work is this. The homeowner almost never appoints anybody. They usually have no idea they are supposed to. Because they are a domestic client, the duties do not bounce back to them the way they would on a commercial job. They land, automatically and by operation of the regulations, on whoever is in control of the work — which, on most domestic jobs with more than one trade, is you.

You do not have to agree to it. You do not have to be told. If you are the contractor in control of the construction phase, you are the Principal Contractor.

This article is guidance, not legal advice. Regulations 5, 6 and 7 quoted here were checked word-for-word against the published text on legislation.gov.uk on 23 September 2026. Always check the current wording, or speak to a competent CDM adviser, before relying on it for a live job.