Principal Designer vs Principal Contractor: Who Does What Under CDM 2015

Under CDM 2015, the Principal Designer runs health and safety while the job is still being designed, and the Principal Contractor runs it once boots are on site. On any project with more than one contractor, the client appoints both in writing, and if you skip that appointment the duties fall on you.

Two roles, two phases.

We sit on the Principal Designer side of that line most weeks, usually on refurbishment and town-centre schemes around Stockport and the rest of Greater Manchester. The muddle between these two roles is the most common CDM question a developer asks us, so here is the plain version, with the parts that actually catch people out.

Who appoints the Principal Designer and the Principal Contractor?

You do, if you are the client. Regulation 5 of CDM 2015 says that on any project involving more than one contractor, the client must appoint a Principal Designer and a Principal Contractor, and the appointment has to be in writing. One contractor only? Then you still carry CDM duties, but you do not need the two principal roles.

Here is the part people miss. If you do not make those appointments, the law does not leave a gap. The client is treated as holding the duties personally. So a developer who never formally appointed a Principal Designer has not dodged the role. They have taken it on themselves, usually without the competence or the professional indemnity cover to carry it. That is a bad place to be standing if the HSE turns up.

If you are lining up a scheme and are not sure which appointments you are legally on the hook for, our CDM advisor and Principal Designer team will map it against your procurement route before you commit.

What does the Principal Designer actually do?

The Principal Designer controls health and safety through the pre-construction phase. That is the design and planning stage, before the build and often well into it, wherever design is still being developed. The job is to plan, manage, monitor and coordinate health and safety while decisions are still on paper, because that is where the cheapest and safest fixes live.

In practice we are designing risk out before anyone can be hurt by it. Can the plant room be reached for maintenance without a cherry picker parked over a live road. Is that atrium detail going to force someone to work at height for a fortnight when a different junction would not. We also gather the pre-construction information the client holds and get it to the design team and the contractor, and we open the health and safety file that gets handed over at the end.

The Principal Designer must be a designer with real control of the design work, an architect or engineer with the competence to manage safety inside it. That is a deliberate change from the old CDM coordinator role, which CDM 2015 scrapped when it came into force on 6 April 2015.

What does the Principal Contractor actually do?

The Principal Contractor takes the same four verbs, plan, manage, monitor and coordinate, and applies them to the construction phase on site. Site rules, inductions, keeping trades from working on top of one another, welfare, traffic management, checking the scaffold gets inspected. If the Principal Designer owns the risk on the drawing, the Principal Contractor owns the risk in the mud.

They also write and hold the construction phase plan, which every project needs before work starts, right down to a single-contractor domestic job where the lone contractor writes it. And they keep the health and safety file current as the build throws up changes, then hand it back for the client at the end.

Principal Designer vs Principal Contractor, side by side

Can the same firm be both Principal Designer and Principal Contractor?

Yes, and on a design-and-build contract it is common for one organisation to hold both. CDM does not forbid it. But I will say plainly what some design-and-build contractors will not: putting both roles under one roof removes a natural check. The Principal Designer is meant to be an independent voice on buildability and safety in the design, and it is harder to challenge a construction programme when your own company is trying to hit it.

We manage that tension when we hold both by keeping the design-safety review genuinely separate from the site team. Where a client wants a fully independent Principal Designer, we act as exactly that and push back on the contractor's design where it needs it. Neither route is wrong. Pretending the conflict is not there is how it bites you later.

An honest one from our side. We picked up a Greater Manchester refurbishment where the pre-construction information handed to us was nearly three years old, and the asbestos register came with it. We stopped and commissioned a fresh refurbishment and demolition survey under the Control of Asbestos Regulations 2012 before we would let the design proceed, and that pushed the start back by about 5 weeks. We would rather carry that delay than sign off a pre-construction phase on a register we did not trust. Anything built before 2000 in this part of the country is an asbestos question until a survey says otherwise, and a fair amount of Stockport town-centre stock is a good deal older than that.

Do you have to notify the HSE, and who does it?

Sometimes, and it is the client's legal duty, though the Principal Designer usually prepares it. You notify the HSE on form F10 if the construction work is going to last longer than 30 working days and have more than 20 workers on at the same time at any point, or if it will exceed 500 person-days of work. Cross either threshold and the F10 goes in before construction starts.

Plenty of commercial refurbishments clear that bar without anyone clocking it until late. A fit-out running four months with 25 trades on at peak is notifiable, and a missing F10 is an easy thing for an inspector to pick up on.

Is the CDM Principal Designer the same as the one under the Building Safety Act?

No, and this is the trap that catches people in 2026. Since 1 October 2023 there has been a second, separate Principal Designer role, created under the Building Safety Act 2022 and sitting inside the Building Regulations. It is about compliance with the building regulations, competence and design quality, not the CDM health and safety duties.

So a single project can need two Principal Designer appointments, doing different jobs under different law, and they are not automatically the same firm. On higher-risk buildings, broadly those at least 18 metres tall or with 7 or more storeys, the regime is tighter again. If a consultant talks about the Principal Designer as one role, ask which one they mean. We keep the two appointments explicit on every job so nothing drops down the gap between them.

Getting the appointments right on your scheme

The short version. Designer-side safety before site is the Principal Designer. Site-side safety during construction is the Principal Contractor. You appoint both in writing when more than one contractor is involved, you notify the HSE if the F10 thresholds are met, and you keep the CDM role separate in your head from the Building Safety Act role that happens to share its name.

We act as Principal Designer and CDM advisor across the North West, and we work alongside your contractor on construction-phase health and safety, independent quality assurance and clerk of works, and full construction project management where you want one team holding the whole thing together. If you would like us to look at your project and tell you which CDM appointments you need, tell us about the scheme and we will come back to you.