CDM Regulations for Home Extensions in London: A Homeowner's Guide
The Construction (Design and Management) Regulations 2015 apply to virtually every home extension in London. For most homeowners, the obligations are manageable — but understanding what they are, who does what, and what documents you need to keep will save you problems during the build and when you come to sell.
Quick Answer
CDM 2015 applies to all home extensions, but domestic clients have reduced duties compared to commercial clients. Most standard London extensions are not notifiable to the HSE. Where one main contractor is used, CDM duties largely sit with the contractor. Where multiple contractors are involved, you should appoint a Principal Designer and Principal Contractor in writing. The key document you need at the end is the Health and Safety file.
500+ person-days
Notifiable threshold
20+ workers
Or: 30 days +
Not notifiable
Most extensions
Check your specific property constraints
Free Property CheckWhat Are CDM Regulations?
The Construction (Design and Management) Regulations 2015 — universally known as CDM 2015 — are the primary piece of health and safety legislation governing construction projects in Great Britain. They came into force on 6 April 2015, replacing the earlier 2007 version, and they apply to all construction work: commercial projects, public works, and private domestic projects alike.
The purpose of CDM 2015 is to improve health and safety during construction by making key parties think carefully about risk at the design stage, plan for it before work starts, and manage it properly on site. The regulations are enforced by the Health and Safety Executive (HSE).
For homeowners building extensions in London, the headline message is this: CDM 2015 applies to your project, but your obligations as a domestic client are significantly lighter than those placed on commercial clients. In most cases, a standard rear or side extension uses a single main contractor, and the bulk of the CDM duties sit with that contractor and your architect — not with you personally.
The reassuring reality: For a typical London single-storey rear extension with one main contractor and a structural engineer, CDM compliance largely means ensuring your contractor has a Construction Phase Plan, keeping the Health and Safety file at the end of the project, and providing any known site information upfront. Your architect and contractor should manage the rest.
What Is a Domestic Client?
CDM 2015 defines a domestic client as a person who has construction work carried out on their own home, or on the home of a family member, where it is not done in connection with any business. If you are a London homeowner commissioning an extension to your own property, you are a domestic client under CDM 2015.
The significance of this is that domestic clients are exempt from several of the duties that apply to commercial clients. Specifically, a domestic client does not need to:
- Formally carry out the full client duties under Regulation 4 (managing projects)
- Submit F10 project notification to the HSE themselves (though notification may still be required — see below)
- Ensure welfare facilities are provided (this duty passes to the contractor)
However, domestic clients are not entirely removed from CDM. The rules are structured so that when a domestic client does not actively take on the management responsibilities, those responsibilities transfer automatically to other duty holders — typically your contractor or principal designer. The duties still need to be performed; they just do not rest with you personally unless you choose to take them on.
How duties transfer for a domestic client
Where only one contractor is involved (the most common scenario for a standard extension), the contractor automatically takes on the client's CDM duties. You do not need to appoint anyone separately.
Where multiple contractors are used (for example a main builder, a separate electrical contractor, and a separate plumber all working under your direct instruction rather than under a main contractor), you should appoint a Principal Designer and Principal Contractor in writing. If you do not make these appointments, the first contractor you appoint automatically takes on the Principal Contractor duties.
A domestic client can enter into a written agreement with the Principal Designer for that designer to take on the client's CDM duties as well as their own. This is often the most practical arrangement for a homeowner who wants to ensure full compliance without having to manage the process themselves.
Quick Answer
For a standard London rear or side extension with a single main contractor, CDM compliance is largely handled by your architect and contractor. The key actions for you as a homeowner are: share any known site information before work starts, ensure your contractor produces a Construction Phase Plan, and keep the Health and Safety file when the project completes.
Duties transfer to them
One contractor
Appoint PD + PC
Multiple contractors
Not notifiable
Most extensions
Check your specific property constraints
Free Property CheckCDM Duty Holders: Who Does What
CDM 2015 defines five duty holders: the Client, the Principal Designer, the Principal Contractor, Designers, and Contractors. On a typical London home extension, these roles are filled by a small group — you, your architect, and your builder — and the responsibilities are largely practical rather than administrative.
Client (you)
As a domestic client, your core obligations under CDM 2015 are:
- Provide pre-construction information to your designer and contractor (details of the existing property, known hazards, service locations)
- Ensure suitable arrangements are in place for managing the project (in practice: appoint competent parties)
- Keep the Health and Safety file once the project is complete
Principal Designer (your architect or lead designer)
On a project involving more than one contractor, the Principal Designer role is usually taken by your architect. Their CDM duties include:
- Plan, manage, monitor, and coordinate health and safety during the pre-construction phase
- Identify, eliminate, and manage design risks — designing out hazards where possible
- Collate and maintain the pre-construction information pack
- Prepare and hand over the Health and Safety file at project completion
- Ensure all designers consider construction phase safety in their designs
Not all architects automatically accept the Principal Designer role under CDM — confirm this is in your appointment letter.
Principal Contractor (your main builder)
The Principal Contractor is responsible for health and safety during the construction phase. Their CDM duties include:
- Produce and implement the Construction Phase Plan before work begins
- Manage and coordinate all contractors working on site
- Ensure site welfare facilities are adequate
- Consult with workers on health and safety matters
- Contribute information to the Health and Safety file
Designers
Any professional who produces design information for a construction project — architects, structural engineers, interior designers — is a Designer under CDM 2015. Designers must consider how to eliminate or reduce health and safety risks through their design decisions, rather than simply documenting what the project will look like. This means a structural engineer specifying beam connections must consider how those connections can be made safely on site, not just whether the structure will be sound when complete.
Contractors
Any individual or business carrying out or managing construction work on site is a Contractor under CDM 2015. On a standard extension, this includes the main builder and any specialist subcontractors (electricians, plumbers, roofers) whether engaged by the main contractor or directly by you. Each contractor must plan, manage, and monitor their own work to ensure it is carried out safely and in accordance with the Construction Phase Plan.
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Is Your Extension Notifiable to the HSE?
The question homeowners most often ask about CDM is whether they need to notify the Health and Safety Executive (HSE) about their project. The short answer for most London home extensions is: no.
Under CDM 2015, a project becomes notifiable to the HSE only if it meets either of the following thresholds:
Notifiable project thresholds
The construction phase will last longer than 30 working days, and at some point will have more than 20 workers working simultaneously on site.
The construction work will exceed 500 person-days in total. A person-day is one person working for one day (or the equivalent in part-days). A team of five workers on site for 100 days equals 500 person-days.
In practice, the vast majority of standard London home extensions do not come close to these thresholds. A typical single-storey rear extension takes 12–16 weeks to build with a site team rarely exceeding six or eight people. The total person-days will be well under 500, and there will not be 20 workers on site simultaneously at any point.
Larger, more complex projects could potentially reach the threshold: a basement extension combined with a rear extension on a large house, involving intensive excavation, underpinning, structural steel installation, and multiple specialist trades all working in sequence over many months, might approach or exceed 500 person-days. If you have any doubt, ask your architect to calculate the likely person-days before work starts.
If your project is notifiable: The F10 notification must be submitted to the HSE before the construction phase begins. As a domestic client, the notification is submitted by your Principal Contractor (or your Principal Designer if they have agreed to take on client duties). You do not need to submit it yourself. The HSE's online F10 portal is at hse.gov.uk. The notification cannot be undone — it signals to the HSE that this is a project they may inspect.
CDM still applies even when not notifiable
It is important to understand that CDM 2015 applies to all construction projects, regardless of whether they are notifiable. A project that falls below the notification thresholds still requires a Construction Phase Plan, duty holders, and a Health and Safety file. The notification threshold does not determine whether CDM applies — it only determines whether you must formally alert the HSE.
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Check my project for free →Pre-Construction Information: What You Need to Provide
Before any design or construction work begins, the client must provide relevant pre-construction information to the designer and contractor. For domestic clients, this is the most practical and hands-on CDM obligation you will face.
The pre-construction information is essentially everything you know about your property that could affect the safety of the design or the build. Your Principal Designer will typically ask you for this at the start of the project and will collate it into a Pre-Construction Information Pack.
Common pre-construction information items
You do not need to commission new surveys or investigations to satisfy CDM — you simply need to share whatever you already know or have. If you do not have drawings or surveys, your architect will typically arrange what is needed as part of the design process. The obligation is to share what exists, not to create information from scratch.
The Health and Safety File
The Health and Safety file is the document that CDM 2015 requires the client to keep after the project is complete. It is one of the most practically important CDM outputs for homeowners — not because the HSE will ever ask to inspect it, but because it contains information that future contractors, architects, and surveyors will need when they work on the property.
The Principal Designer is responsible for preparing and handing over the Health and Safety file. If the Principal Designer's appointment ends before the project is complete, responsibility for completing the file passes to the Principal Contractor. At the end of the project, the file should be passed to you as the client to keep for the life of the structure.
What goes in the Health and Safety file
Why this matters when you sell: Solicitors conducting property conveyancing will ask for building regulations documentation and any relevant construction records for work done on the property. A complete Health and Safety file provides this evidence in a single document. Properties where this documentation is missing or incomplete can cause delays during the sale process.
In practice, on small domestic projects, the Health and Safety file is sometimes a modest folder of A3 drawings, an O&M (operations and maintenance) manual from the contractor, and some product data sheets — rather than an elaborate document. The important thing is that it is produced, complete, and that you keep it safely for the life of the building.
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The Construction Phase Plan
Before construction starts, the Principal Contractor must produce a Construction Phase Plan. This document sets out how the construction phase will be managed safely. Unlike the Health and Safety file (which is primarily for future reference), the Construction Phase Plan is a live document used throughout the build.
The level of detail in the plan should be proportionate to the size and risk of the project. For a straightforward single-storey extension, the plan might be a few pages covering site logistics, traffic management, welfare arrangements, emergency procedures, and the specific risks associated with the project (excavation, scaffold, demolition of an existing structure).
Typical Construction Phase Plan contents
As the client, you do not write the Construction Phase Plan — your contractor does. But you should ask to see it before work starts, and your architect should review it as part of their Principal Designer duties where applicable. If your contractor cannot produce a Construction Phase Plan or dismisses the requirement, this is a warning sign about their competence and professionalism more broadly.
Practical CDM Checklist for London Homeowners
For a standard London home extension, the following steps will ensure CDM compliance without adding significant cost or complexity to the project:
Confirm CDM roles at appointment stage
When appointing your architect, confirm in writing whether they are accepting the Principal Designer role under CDM 2015. When engaging your contractor, confirm they will act as Principal Contractor. These confirmations should be in your appointment letters or contracts — not just verbal agreements.
Provide all relevant site information upfront
Share any drawings, surveys, reports, and utility information you have about the property as early as possible. If the extension will involve opening up walls or excavating near existing services, commission a utility survey rather than relying on memory or assumption. This information forms your pre-construction information contribution under CDM.
Ask for the Construction Phase Plan before work starts
Your contractor should provide a Construction Phase Plan before any work begins on site. You do not need to approve it — that is the Principal Designer's job — but you should have seen it. A contractor who cannot produce one should prompt a conversation about their CDM competence.
Commission an asbestos survey if the property pre-dates 2000
If your London property was built before 2000 and the extension involves disturbing any existing structure, an asbestos management survey should be carried out before work starts. Asbestos can be found in artex ceilings, floor tiles, roof felt, pipe lagging, and insulating board in older properties. The survey result forms part of the pre-construction information and the Construction Phase Plan must address it.
Request and keep the Health and Safety file
At the end of the project, your Principal Designer (or contractor if there is no Principal Designer) must hand over the Health and Safety file. Ask for it explicitly, keep it safely, and store it alongside your planning consent, building regulations completion certificate, and other property documents. A digital copy is advisable as well as a physical folder.
Check whether your project is notifiable
Ask your architect at the outset to calculate the expected total person-days for the project. For a standard extension, the answer will be well under 500 and the project will not be notifiable. If the project is more complex, this check ensures you do not inadvertently start a notifiable project without an F10 being submitted.
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Get my free estimate →Frequently Asked Questions
Do CDM Regulations apply to my home extension?
Yes. CDM 2015 applies to all construction work in Great Britain, including domestic home extensions. However, as a domestic client, your personal obligations are limited compared to commercial clients. For most standard extensions where a single main contractor is used, the CDM duties transfer automatically to your contractor. You are primarily responsible for providing pre-construction information and keeping the Health and Safety file at the end of the project.
Do I need to notify the HSE about my home extension?
Almost certainly not. A project only needs to be notified to the HSE if it will last longer than 30 working days with more than 20 workers simultaneously on site, OR if total construction work will exceed 500 person-days. The vast majority of London home extensions — a typical rear or side extension takes 12–16 weeks with a small team — fall well below these thresholds. Ask your architect to confirm at the start of the project.
What is the F10 notification form?
The F10 is the form used to notify the HSE of a notifiable construction project. It includes details of the client, Principal Designer, Principal Contractor, project location, and the expected construction programme. For domestic clients on notifiable projects, the F10 is submitted by the Principal Contractor or Principal Designer — not by the homeowner personally. The HSE's online F10 portal is at hse.gov.uk/forms/notification/f10.htm.
Do I need a Principal Designer for my home extension?
On a project involving more than one contractor, you should appoint a Principal Designer. For a standard extension where a single main contractor manages all the works (including any specialist subcontractors), a formal Principal Designer appointment is less critical, as the contractor takes on the CDM duties. In practice, your architect will often take on the Principal Designer role — but this should be confirmed in writing in their appointment letter.
What is the Health and Safety file and why do I need it?
The Health and Safety file is a document produced at the end of the project by the Principal Designer or contractor. It records as-built drawings, structural information, service locations, materials used, maintenance requirements, and any residual hazards for future contractors to be aware of. As a homeowner, you keep it for the life of the building. It is important when you come to sell, as solicitors will request documentation for building works, and it protects future contractors working on the property.
What is pre-construction information under CDM?
Pre-construction information is anything you know about your property that is relevant to the safe design and construction of the extension. This includes existing drawings, structural surveys, locations of underground services, asbestos survey results, and party wall documentation. Your obligation as a client is to share what you have — you do not need to commission new surveys to satisfy CDM, though your architect may recommend surveys as part of the normal design process.
Summary
CDM 2015 applies to your London home extension, but for most homeowners the practical obligations are straightforward. Appoint competent parties — confirm your architect will act as Principal Designer and your contractor as Principal Contractor. Share what you know about the property before work starts. Ask to see the Construction Phase Plan. Keep the Health and Safety file when the project is complete.
The notification threshold — 30 working days with 20 or more workers simultaneously on site, or 500 person-days — is well above what a standard London extension involves. You are unlikely to need an F10 notification for a rear or side return extension. Larger projects such as basement conversions or whole-house refurbishments may come closer to the threshold, and it is worth checking with your architect.
The most important things CDM asks of you are not onerous: share information, appoint competent professionals, and keep records. The regulations exist to reduce serious injuries and deaths on construction sites — not to create administrative burdens for London homeowners doing sensible home improvements.
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