For developers and business owners procuring a build
Principal Contractor vs Main Contractor: What the Difference Means for Your Project
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 10 min read
The short answer
A principal contractor is a legal role under the CDM Regulations 2015: the contractor the client appoints in writing to control health and safety on site. A main contractor is a commercial role, the firm that holds the building contract, and on most projects one company is both.
- With more than one contractor on a project, the client must appoint a principal contractor before work starts.
- No appointment means the client carries the principal contractor's duties personally.
- Since 1 October 2023 there is a second principal contractor role under the Building Regulations, for compliance, not just safety.
- The main contractor's duties come from the contract; the principal contractor's from regulations 12 to 14.
Principal contractor vs main contractor: the first is a legal role, the second a commercial one. The principal contractor is the contractor the client appoints in writing, under the CDM Regulations 2015, to control health and safety during the construction phase. The main contractor is the firm that holds the building contract and delivers the works.
On most projects the client appoints its main contractor as principal contractor. But until that appointment is made, the main contractor is only the main contractor. The gap matters because the duties do not wait. If nobody is appointed, the client holds the principal contractor's duties itself.
And since 1 October 2023 there is a second principal contractor under the Building Regulations, with its own duties.
This guide sets out who holds what, when the appointments are made and what the duties look like on site. We take both appointments on our projects and run construction project management for larger schemes. Appointing a contractor now? Book a site visit and we will show you how we would set the roles up.
Principal contractor vs main contractor: the three CDM roles
CDM 2015 names five dutyholders: clients, designers, principal designers, contractors and principal contractors. Three of them decide how a project is run. "Main contractor" is not one of them. The table shows where each role comes from and what it is responsible for.
| Role | Where it comes from | What it is responsible for |
|---|---|---|
| Client | CDM 2015, regulation 4 | Suitable arrangements, time and resources; pre-construction information; appointing the two principals in writing; checking they do their duties |
| Principal designer | CDM 2015, regulations 11 and 12 | Planning, managing and monitoring the pre-construction phase; design risk; the health and safety file |
| Principal contractor | CDM 2015, regulations 12 to 14 | Planning, managing and monitoring the construction phase; the construction phase plan; site induction, security and welfare |
| Main contractor | The building contract | Delivering the works to the contract: price, programme, quality, defects. No CDM duties beyond those of any contractor, unless appointed principal contractor |
The main contractor's duties are commercial: they come from the JCT or other contract you sign. The principal contractor's duties are statutory: they come from the regulations and apply whatever the contract says. That is why the appointment has to be made in writing, by the client, and not left to the contract to imply.
Our guide to CDM 2015 client duties covers the client's side in detail. This article stays on the contractor's side of the line.
When a principal contractor must be appointed, and by whom
Regulation 5 sets the rule. Where more than one contractor is working on a project, the client must appoint a principal designer and a principal contractor in writing. The same applies where it is reasonably foreseeable that more than one will. The appointments are made as soon as practicable, and in any event before the construction phase begins.
"More than one contractor" is a low bar. A main contractor with one subcontracted electrician is two contractors. Almost every commercial project in London qualifies.
Only the client can make the appointment. A main contractor cannot appoint itself, and an architect cannot do it on the client's behalf unless the client has put that in writing.
The appointment sequence on a real project
On a real project the sequence looks like this.
- Pre-construction. The client appoints the principal designer as soon as design work starts, so design risk is managed from the first drawing.
- Tender. The tender documents say the successful contractor will be appointed principal contractor. Each bidder is checked for the skills, knowledge, experience and organisational capability regulation 8 requires.
- Award. The client appoints the main contractor as principal contractor in writing. Ours is a letter or a clause in the contract, dated before the site is set up.
- Before site set-up. The principal contractor draws up the construction phase plan. The client checks it exists before the construction phase begins, which regulation 4 requires.
- Notification. If the project passes the threshold below, the client submits the F10 notice to HSE before the construction phase begins.
When the project is notifiable
A project is notifiable if the work is scheduled to last longer than 30 working days with more than 20 workers on site at once. It is also notifiable if the work will exceed 500 person-days. HSE only accepts the F10 online, and a copy goes up in the site office.
Notification is a record, not a trigger: the duties above apply to every project, notified or not.
If the client does not appoint a principal contractor, regulation 5(4) is blunt. The client must fulfil the principal contractor's duties in regulations 12 to 14 itself. For a developer or a business owner, that means owning the construction phase plan, site security and welfare on a site it does not control.
Principal contractor duties in practice (regulation 13)
Regulation 13 gives the principal contractor one job in a sentence. Plan, manage and monitor the construction phase, and coordinate health and safety, so that work is carried out without risks to health or safety. The rest of regulations 12 to 14 says how. On site, it looks like this.
- The construction phase plan. Written before the site is set up. It sets out the arrangements and site rules, with specific measures for the higher-risk work in Schedule 3. It is reviewed as the job changes.
- Cooperation between contractors. The principal contractor organises how the trades work together and in sequence, and makes sure each applies the general principles of prevention.
- Site induction. Everyone who comes onto the site gets a suitable induction before they start.
- Site security. Reasonable steps to keep unauthorised people out, which in a trading building or a shared street means hoarding, access control and a signed-in site.
- Welfare. Toilets, washing, rest and changing facilities that meet Schedule 2, in place from the first day and kept through the construction phase.
- Liaison with the principal designer. Design changes during the build go back to the principal designer, and the information for the health and safety file flows the other way.
- Consulting the workforce. Regulation 14 requires workers to be consulted in good time on matters that affect their health, safety or welfare.
None of this is paperwork for its own sake. The construction phase plan is the document that tells a new subcontractor how the site runs. The induction is the ten minutes that stop someone walking into a live electrical riser. A contractor who treats these as box-ticking usually treats the programme the same way.
The Building Regulations principal contractor since October 2023
The Building Safety Act 2022 added a second dutyholder regime. Part 2A of the Building Regulations 2010 came into force for England on 1 October 2023. It uses the same titles, client, principal designer and principal contractor, for a different purpose: compliance with the Building Regulations, not safety on site.
The trigger is the same. Where more than one contractor will work on a project, the client must appoint a principal designer and a principal contractor in writing. That happens before the construction phase begins. If not, the client carries the duties until it does. Where there is only one contractor, that contractor is treated as the principal contractor.
Two points make this workable rather than a second layer of paperwork.
- One firm can hold both roles. The CDM principal contractor can also be the Building Regulations principal contractor. The client confirms this in writing, and under regulation 11D it is then treated as having made the appointment.
- The duties are about compliance. The Building Regulations principal contractor must plan, manage and monitor the building work and coordinate all contractors so the work complies with the regulations. It must never accept non-compliant work. When its appointment ends, it has 28 days to give the client a document explaining the arrangements it put in place.
Competence and higher-risk buildings
Competence is written into the role. Under regulation 11H, a company acting as principal contractor must designate an individual to manage the function. It must satisfy itself that this person has the skills, knowledge, experience and behaviours to do it. When you appoint a contractor, ask who that individual is.
Higher-risk buildings carry further duties. These are buildings of at least 18 metres or seven storeys with at least two residential units. Building control approval must come from the Building Safety Regulator before work starts. That is outside this article, but the principal contractor role is the same one.
We recently hired Craftex for a commercial office fit-out project in Central London, and overall, we’re really happy with the result. There was a delay on our side with a custom glass partition supplier, but the Craftex team managed the construction schedule very professionally and kept everything on track. Their guys worked clean, respected our workspace and IT setup, and kept the site tidy throughout. If you're looking for a reliable office refurbishment company in London, I’d definitely recommend Craftex. Would use them again without hesitation.
Design and build or traditional: who holds which role
Procurement decides which firm sits in each chair. The CDM roles do not change with the contract; the people who fill them do.
| Role | Traditional procurement | Design and build |
|---|---|---|
| Principal designer (CDM) | The architect or a specialist CDM adviser appointed by the client | Often the design and build contractor, once it controls the design; the client's designer before novation |
| Principal contractor (CDM) | The main contractor, appointed at award | The design and build contractor, appointed at award |
| Main contractor | The firm that wins the tender and builds to the client's design | The firm that takes on the design and builds it |
| Contract administration | Architect or contract administrator certifies | An employer's agent acts for the client |
Under design and build, one firm can be principal designer and principal contractor at the same time. It needs control over each phase and the competence for both. That removes the handover between the two roles, which is where pre-construction information and design-risk decisions get lost.
The client's own adviser on that route is the employer's agent, who administers the contract but holds none of the CDM roles.
Under traditional procurement, the roles are split. Our commercial architects London team works on both routes. In each case the client's question is the same: does the firm you are appointing control the phase it is being made responsible for?
Choosing a main contractor in London: what to check
The main contractor you appoint will almost always be your principal contractor as well. So the checks you make before award are CDM checks as much as commercial ones. Regulation 8 requires the client to take reasonable steps to confirm the contractor has the skills, knowledge, experience and organisational capability for the role.
In practice, we would expect a client to ask a London contractor for the following.
- A construction phase plan from a comparable project. Not a template. A real one, with site-specific rules, shows whether the firm plans a site or copies a document.
- The named individual who will manage the principal contractor function. Regulation 11H requires one. If the firm cannot name them at tender, the role is not staffed.
- Who does the work. A firm with its own trades controls induction, sequence and quality directly. One that subcontracts everything is coordinating strangers.
- Insurance and accreditations. Public and employers' liability at a level that matches the project, and current health and safety accreditation.
- The programme record. Ask what share of recent projects finished on or ahead of programme, and ask to speak to one of those clients.
- The welfare and security plan for your site. On an occupied building or a tight street, this is where the real risk sits, and where a weak contractor shows early.
Our list of the top commercial construction companies in London shows how the larger firms compare. For an office fit out London project, the same questions apply at a smaller scale. The building is occupied, the landlord is watching and the lease date does not move.
What this means for your project
As main contractor on our projects, Craftex takes the principal contractor appointment under CDM 2015 and the Building Regulations. It is made in writing, before the site is set up. Construction is an in-house department. Around 60% of our labour is our own, so induction, sequence and the construction phase plan are controlled by the people doing the work.
We have worked this way since 2016, and every project has finished ahead of programme and on budget. Our Operations Director has previously run projects up to £135M. We take on construction projects from £100,000 to £10M and above, and client-side construction project management for schemes from £5M.
If you are about to appoint a contractor for a London project, book a site visit. We will walk the building with you and tell you what the construction phase plan needs to cover. You get a programme and a price before you commit.
Before you appoint
- More than one contractor on the project, so both principal roles are required
- Principal designer appointed in writing at the start of design
- Principal contractor appointed in writing at award, before site set-up
- Same firm confirmed in writing for the Building Regulations roles, if that is the intent
- Construction phase plan in hand before the construction phase begins
- F10 submitted if the project is notifiable
- Named individual managing the principal contractor function
- Pre-construction information handed over: surveys, asbestos, services, structure
