Construction compliance guide
Do I Need a Construction Phase Plan for a Domestic Extension?
A plain-English guide to CPP requirements for house extensions under CDM 2015. When it’s legally required, what the domestic client rules mean for you, and how to produce one without the headache.
TL;DR
- • Yes, you need a CPP for a domestic extension — CDM 2015 in Great Britain applies to all construction work, with no project-size exception.
- • On a domestic job with only one contractor, the homeowner’s CDM client duties transfer automatically to that contractor.
- • The CPP should reflect the work and risks; no fixed page count proves it contains the required arrangements.
- • F10 notification is only required if the project lasts more than 30 working days with more than 20 workers at any one time, or exceeds 500 person-days — most extensions won’t hit that.
- • Not having a CPP is a legal breach that can result in enforcement action, fines, or prosecution by the HSE.
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What does the law actually say?
The Construction (Design and Management) Regulations 2015 — known as CDM 2015 — apply to all construction work in Great Britain. There is no minimum project value, no size threshold, and no exemption for domestic work. If you are carrying out construction work, CDM 2015 applies.
Regulation 15(5) is the one that matters on most extensions: where you are the only contractor on the job, you must draw up a construction phase plan — or arrange for one to be drawn up — as soon as is practicable before the site is set up. Where there is more than one contractor, that same plan is the principal contractor’s duty under Regulation 12(1). The wording is clear — it is not optional, it is not “if reasonably practicable,” and it is not limited to commercial projects. The construction phase plan must be in place before the first day on site.
Construction work is defined broadly in the regulations. It includes building, alteration, fitting-out, commissioning, renovation, repair, upkeep, decoration, demolition, and dismantling. A house extension falls squarely within this definition. Whether you are building a single-storey kitchen extension, a two-storey side return, or a loft conversion with a dormer, you are doing construction work and you need a CPP.
How do the domestic client rules work?
CDM 2015 recognises that a homeowner having an extension built is in a very different position to a property developer commissioning a block of flats. That is why the regulations include special provisions for domestic clients.
A domestic client is a person who has construction work carried out on their own home, or the home of a family member, and the work is not done in connection with a business. Most homeowners commissioning extensions, loft conversions, and renovations fall into this category.
Under Regulation 7, domestic clients do not have to carry out CDM client duties themselves. Instead, those duties transfer automatically away from the client. By default, if there is only one contractor on the project, the regulation 4 and 6 client duties land on them. If there is a principal contractor appointed on a project with more than one contractor, the duties transfer to the principal contractor by default instead.
On a domestic project with only one contractor, that contractor holds the client duties by default, including ensuring a CPP is produced and suitable welfare is available. Where there is more than one contractor, those client duties sit with the principal contractor by default, or, where there is more than one contractor, instead with a principal designer who agrees in writing to fulfil them. The domestic client separately keeps the regulation 5 appointment duty, which arises only where there is more than one contractor.
There is one exception to the default: where there is more than one contractor, a domestic client can make a written agreement with a principal designer to transfer the client’s CDM duties instead — including the duty to notify the HSE. On a job with only one contractor there is no principal designer to make that agreement with, so those duties stay with that contractor. Where such an agreement is made, ALL of those duties transfer to the principal designer, not just some of them. The competence check transfers too: regulation 8(3) requires whoever appoints a designer or contractor to take reasonable steps to satisfy themselves that they have the skills, knowledge and experience for the role, and regulation 8(7) places that duty, so far as it would otherwise fall on a domestic client, on whoever holds the client duties under regulation 7(1) — not on the homeowner. Where there is more than one contractor, that written agreement does of course need the principal designer to agree to it. There is no equivalent duty-transfer agreement for a principal contractor; a principal contractor already receives these duties by default whenever there’s more than one contractor — though, like a principal designer, a principal contractor must still be appointed in writing on a project with more than one contractor, under regulation 5, separately from that question. If the client never makes that appointment on a job with more than one contractor, regulation 7(2) deems whichever contractor is in control of the construction phase to be the principal contractor instead — a sole contractor instead holds the duties directly, with no principal contractor role to deem. Appointing a principal designer this way is uncommon on small domestic jobs but occasionally happens on larger extensions where an architect is already involved.
When is a CPP legally required for an extension?
The short answer is: always. Every construction project needs a Construction Phase Plan before the construction site is set up. There is no lower threshold. A single-storey rear extension needs one. A loft conversion needs one. Even a straightforward bathroom refurbishment technically needs one.
The common confusion arises because builders mix up the CPP requirement with the F10 notification requirement. Notifying the HSE (via an F10 form) is only required for projects that exceed certain size thresholds. The CPP, by contrast, is required for every project regardless of size.
The fact that your project is not notifiable does not mean you do not need a CPP. These are two separate requirements under CDM 2015. Every construction project needs a CPP, while regulation 6 notification applies only when a project meets an F10 threshold.
The CPP must be in place before the construction site is set up. Draw it up or arrange for it to be drawn up during preparation, before your team arrives on site. Writing it retrospectively misses the required planning stage.
Do I need to notify the HSE (F10)?
F10 notification is required under Regulation 6 of CDM 2015 when a project meets either of two thresholds:
- The construction phase will last longer than 30 working days AND more than 20 workers will be on site at any one time.
- The project will exceed 500 person-days of construction work.
Let’s put that in context for a typical domestic extension. A rear kitchen extension with a team of 3–4 workers might take 8–12 weeks to complete. That is more than 30 working days, but you are nowhere near 20 workers on site simultaneously. And 4 workers over 12 weeks is roughly 240 person-days — well below 500.
Most domestic extensions will not trigger F10 notification. But remember: even if F10 is not required, the CPP still is. The HSE’s guidance on CDM 2015 makes this distinction clear. If a domestic project meets an F10 threshold, the regulation 6 duty sits with the contractor where there is only one contractor. Where there is more than one contractor it sits with the principal contractor instead, unless a principal designer agrees in writing to fulfil it. The duty holder must notify HSE as soon as practicable before the construction phase begins. They, or someone acting on their behalf, can use the HSE’s F10 notification form.
What should a domestic extension CPP contain?
CDM 2015 does not prescribe a rigid format, but it sets out what the CPP must include. For a domestic extension, the content should be proportionate to the project. You do not need a 30-page document for a kitchen extension — but you do need to cover the essentials.
Project description and key details
The site address, a brief description of the work (e.g. single-storey rear extension with new kitchen), expected start and end dates, and working hours. Include the name of the client (the homeowner), your company name, and the names of any other contractors involved.
Management arrangements
Who is responsible for health and safety on site? On a domestic extension where you are the only contractor, this is straightforward — you are responsible for everything. State this clearly. If you have subcontractors (electrician, plumber, roofer), explain how you will coordinate their work and communicate site rules.
Key risks and how you will manage them
Identify the main hazards on the project. For a typical extension, this might include working at height (scaffolding, roof work), excavations (foundations), manual handling (blocks, lintels), dust and silica (cutting blocks and concrete), electrical safety (temporary supplies), and working near existing services. Describe the control measures for each.
Site rules
Set out the basic rules everyone on site must follow. This includes PPE requirements (hard hat, boots, hi-vis, eye protection), working hours, site access and parking arrangements, smoking and eating areas, and rules about visitors and the homeowner’s access to their property during the works.
Welfare facilities
Describe what welfare provisions are in place. On a domestic extension, this often means using the homeowner’s toilet, washing facilities, and kitchen by agreement. If the homeowner’s facilities are unavailable — for example, if the kitchen is being demolished — you need to arrange alternatives such as a portaloo and hand-washing station.
Emergency procedures
What happens if there is an accident or emergency? Record the site address (so it can be given to 999), the location of the nearest A&E, the name and contact details of the appointed first-aider, and where the first-aid kit is kept. Include fire precautions if hot works are involved.
Fictional example: a typical rear extension
Let’s walk through what a CPP might look like for a common domestic project: a single-storey rear kitchen extension measuring 4m x 6m, with strip foundations, blockwork walls, a flat roof, bi-fold doors, and a full kitchen fit-out.
Project details: Mr and Mrs Smith, 14 Oak Lane, Bristol BS7 8QT. Work duration: 10 weeks. Working hours: Monday to Friday, 08:00–17:00, Saturday 08:00–13:00. Principal contractor: ABC Building Ltd. Subcontractors: an electrician and a plumber (arriving weeks 6–8).
Key risks: Foundation excavation (1.2m deep trench — risk of collapse, falling in, underground services). Working at height during roof construction (scaffolding required, edge protection). Manual handling of dense concrete blocks and steel lintels. Dust from cutting blocks and concrete with a disc cutter. Existing gas main runs along the side of the property — hand dig within 500mm.
Welfare: Homeowners have agreed to provide access to their downstairs toilet and kitchen for tea breaks. A hand-washing station will be set up in the garden for use during groundworks. Drinking water available from an outside tap.
Emergency: First-aider is the site foreman (valid first-aid certificate). First-aid kit kept in the site van. Nearest A&E: Bristol Royal Infirmary, 3.2 miles. Site address given to all workers for emergency calls.
This short sketch illustrates how a builder can record some project-specific arrangements before work starts. Review the full site, scope and risks to decide what the completed CPP needs; its page count alone cannot show whether it is adequate.
How proportionate can a CPP be on a small job?
A CPP is required for construction work, including small jobs. The amount of detail should match the scale, complexity and risks of the work.
Consider a joiner fitting a new front door. That is construction work under CDM 2015, so a CPP is required, but a short plan may be enough. A combined document can serve as both CPP and RAMS if it clearly covers the project health and safety arrangements, site rules, and specific measures for any applicable Schedule 3 work. Regulation 15(6) applies those regulation 12(2) content requirements to a sole contractor’s CPP.
A domestic extension will usually need more detail. You may be digging foundations, working at height, coordinating subcontractors, and managing a live site through the build. The risks are real, the project has complexity, and a CPP is clearly necessary — both legally and practically.
A proportionate CPP also helps in practice: it forces you to think through the risks before starting, it gives you something to show subcontractors during their induction, and it records the planning in case there is an incident. If someone is injured on your site and the HSE investigates, being able to produce a site-specific CPP demonstrates that you took your safety responsibilities seriously.
Common mistakes builders make with domestic CPPs
We see the same mistakes come up again and again come up again and again. Here are the most common ones to avoid:
- Using a generic template without tailoring it to the specific site. The HSE wants to see that you have considered the actual risks at 14 Oak Lane, not risks in general.
- Writing the CPP after the construction site has been set up. The plan must be in place before site setup — regulation 15(5) where you are the only contractor, regulation 12(1) where there is more than one contractor.
- Assuming domestic projects are exempt from CDM. They are not. The domestic client provisions change who holds the duties, not whether the duties exist.
- Forgetting welfare facilities. On a domestic job you might use the homeowner’s facilities, but you need to agree this in advance and record it in the CPP.
- Not updating the CPP when conditions change. If a new subcontractor comes on site, or you discover asbestos in the existing structure, the plan needs updating.
- Confusing the CPP with RAMS. They are different documents. The CPP is the overarching plan; RAMS cover specific tasks.
How long does it take to write a CPP for an extension?
If you are starting from scratch with a blank Word document, a domestic extension CPP means writing every section yourself. That includes thinking through the risks, writing up the welfare arrangements, and making sure you have covered all the required sections. Most of the time is spent staring at a blank page wondering where to start.
A good template helps: it gives you the structure and prompts, so you just need to fill in the site-specific details.
With The Site Book’s Lightweight CPP wizard, you can produce a domestic CPP first draft. Answer 4 questions about your project, and it creates a professional, site-specific plan structured around the CDM 2015 requirements. You review it, tweak anything that needs changing, and download a branded PDF. It is a quick, straightforward way to cover the requirement without cutting corners.
What if there are multiple contractors on a domestic project?
Many domestic extensions involve more than one contractor. You might be the main builder handling the structure, but you bring in an electrician, a plumber, and perhaps a roofer as subcontractors. Under CDM 2015, if there are (or will be) more than one contractor on the project, a principal contractor must be appointed.
On a domestic project with more than one contractor, if no principal contractor is appointed, regulation 7(2) deems the contractor in control of the construction phase — typically the main builder — to be the principal contractor. You do not need a formal written appointment from the homeowner for that to happen. On a domestic project with more than one contractor, the client’s own duties are a separate question: by default they transfer to the principal contractor under regulation 7(1)(b), unless the client has made a written agreement with a principal designer under regulation 7(1)(c).
Where there is more than one contractor, your responsibilities as principal contractor include: producing and maintaining the CPP, coordinating the work of all contractors on site, ensuring every site worker receives a suitable induction, ensuring welfare facilities are adequate, and consulting with workers on health and safety matters. You also need to ensure that only authorised people are allowed on site.
In practice, this means you need to brief your subcontractors when they arrive on site. Show them the CPP, explain the site rules, point out the hazards, and make sure they know where the welfare facilities and first-aid kit are. Keep a dated record of what was covered and who attended. It is useful evidence, although CDM 2015 does not prescribe a signed induction form.
Frequently asked questions
Do I need a CPP for a single-storey rear extension?
Yes. CDM 2015 applies to all construction work in Great Britain, regardless of size. A single-storey rear extension is construction work, so a Construction Phase Plan is required before the construction site is set up. There is no fixed page count: the plan should be proportionate while recording the health and safety arrangements, site rules and specific measures for relevant particular risks.
Does the homeowner need to write the CPP for their own extension?
No. Under CDM 2015, homeowners are classified as domestic clients. Their formal CDM client duties — including the requirement to ensure a CPP is produced — transfer automatically away from the homeowner: by default to the contractor if there is only one contractor (or to the principal contractor if more than one contractor is involved), or, where there is more than one contractor, instead to a principal designer if there is a written agreement to that effect. Either way, the homeowner does not need to write or commission the CPP; the only contractor prepares it on a single-contractor project, while the principal contractor prepares it where there is more than one contractor.
What happens if I start work without a CPP?
Starting construction work without a suitable and sufficient Construction Phase Plan is a breach of CDM 2015 — regulation 15(5) where you are the only contractor on the job, or regulation 12(1) where there is more than one contractor and the duty sits with the principal contractor. HSE may take enforcement action according to the risks and circumstances, including notices or prosecution. Without a CPP, the project lacks its required health and safety planning document.
Do I need to notify the HSE about a domestic extension?
An F10 is required only if the project is scheduled to last longer than 30 working days and have more than 20 workers on site simultaneously, or to exceed 500 person-days. On a domestic project, the regulation 6 duty passes to the contractor where there is only one contractor; where there is more than one contractor it passes to the principal contractor, or, where there is more than one contractor, to the principal designer if there is a written agreement to that effect. Whoever holds the duty must notify HSE as soon as practicable before construction begins. A CPP is required for every construction project whether or not it is notifiable.
Can I use a generic CPP template for every extension I build?
Not without tailoring it. The completed CPP must describe this project's health and safety arrangements, site rules and any relevant Schedule 3 risk measures. A generic template can help you start, but it needs project-specific details such as the work, access constraints, hazards and welfare arrangements before use.
Is a CPP the same as a risk assessment or method statement?
No. A Construction Phase Plan, a risk assessment, and a method statement are three different documents. The CPP is the overarching plan for how health and safety will be managed across the whole project. Risk assessments identify specific hazards and evaluate the level of risk. Method statements describe the step-by-step procedure for carrying out particular tasks safely. Your CPP should reference your risk assessments and method statements, but it is a separate document that sits above them.
CPP on a domestic extension — why 'yes' is the simple answer
Pros
- A domestic CPP should be proportionate to the project; no fixed page count decides whether it contains the required arrangements.
- A site-specific CPP records how health and safety will be managed and gives the project team one clear planning document.
- A written CPP catches site-specific risks (fragile roof, live services, party walls) before work starts.
- On a domestic job with only one contractor, client duties pass to that contractor by default; with more than one contractor, they pass to the PC unless a PD agrees in writing.
Cons
- On a project with only one contractor, starting without a CPP breaches regulation 15(5); where there is more than one contractor, the PC's duty arises under regulation 12(1).
- A template CPP needs site-specific access, welfare and emergency arrangements to be useful.
- A CPP that never gets reviewed after site conditions change (e.g. unexpected asbestos) is effectively out of date.
- On a domestic project, the only contractor must draw up the CPP or arrange for it to be drawn up; with more than one contractor, the principal contractor has that duty.
| Scenario | CPP required? | Who produces it? |
|---|---|---|
| Single-storey rear extension, 1 builder | Yes | The builder (duties transferred from domestic client) |
| Two-storey side extension, 1 builder + subcontractors | Yes | Principal contractor (the main builder) |
| Loft conversion, 1 builder, 4 weeks | Yes | The builder |
| Kitchen refit within existing footprint | Yes (construction work — no exemption) | The builder |
| Garden shed installed by homeowner alone (no contractor) | No (domestic client doing own work) | N/A |
| Major multi-contractor extension scheduled >30 working days + >20 workers simultaneously | Yes + separate F10 notification | PC prepares CPP; on a domestic multi-contractor project the PC holds the F10 duty unless a PD agrees in writing |
“A domestic client's duties automatically pass to the contractor (if it is a single contractor project)”
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What is a CPP?
A plain-English guide to Construction Phase Plans for UK builders.
Domestic Client CDM
How CDM 2015 domestic client duties transfer away from the client by default.
F10 Notification
When and how to notify the HSE about your construction project.
Construction Phase Plan Example
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Sources
- Domestic Clients — HSE · Accessed 17 April 2026