The Construction (Design and Management) Regulations 2015 apply to virtually every building project in the UK — including domestic extensions, loft conversions, and refurbishments. This guide explains who is responsible for what, when HSE notification is required, and how TCM manages CDM compliance on your behalf.
The Construction (Design and Management) Regulations 2015 — known as CDM 2015 — are the main piece of UK legislation governing health and safety on construction sites. They replaced the previous CDM 2007 regulations and brought domestic projects fully within their scope for the first time.
CDM 2015 is enforced by the Health and Safety Executive (HSE). The regulations place legal duties on everyone involved in a construction project — the client, designers, and contractors. The goal is to ensure that health and safety is considered at every stage, from initial design through to the completed building.
For homeowners commissioning a house extension or loft conversion in Hertfordshire, CDM 2015 means your contractor has specific legal obligations around site safety, documentation, and — on larger projects — formal notification to the HSE.
CDM 2015 applies to all construction work in Great Britain, including domestic projects. There is no minimum size threshold for the regulations to apply. However, the specific duties that apply — and whether HSE notification is required — depend on the scale of the project. A single-trade job lasting a few days has far fewer requirements than a multi-trade project running for several months.
CDM 2015 defines five duty holder roles. On a typical domestic extension, the homeowner is the client, TCM acts as Principal Contractor, and our in-house structural engineers and designers fulfil the designer and Principal Designer roles. Here is what each role involves.
Not all projects require formal notification to the HSE. A project becomes notifiable — and therefore requires a Principal Designer and Principal Contractor to be formally appointed — when it meets either of the following criteria.
| Trigger | Threshold |
|---|---|
| Duration | The construction phase will last longer than 30 working days and involve more than 20 workers simultaneously at any point |
| Volume | The construction work will exceed 500 person-days in total |
| Both | Either of the above triggers notification — it is not necessary to meet both |
Most domestic extensions and loft conversions in Hertfordshire do not meet these thresholds. A typical single-storey extension completed by a team of four to six tradespeople over ten weeks involves roughly 200–300 person-days — well below the 500 person-day threshold. TCM will confirm at the survey stage whether your project is notifiable.
A Construction Phase Plan (CPP) must be prepared before construction begins on any project where CDM applies. The Principal Contractor — or the single contractor where there is only one — is responsible for preparing it.
The CPP does not need to be lengthy for a straightforward domestic project. It must, however, cover the key health and safety arrangements for the site: how the site will be managed, welfare facilities, emergency procedures, and how risks specific to the project will be controlled.
TCM prepares a Construction Phase Plan for every project as a matter of course. This is not just a compliance exercise — it is a practical document that helps our site managers coordinate trades safely and efficiently.
On projects with a Principal Designer, a Health and Safety File must be compiled and handed to the client at the end of the project. The file is a record of information relevant to the future maintenance, repair, or demolition of the structure.
A typical Health and Safety File for a domestic extension includes as-built drawings, structural calculations, details of the services installed (electrical circuits, drainage, gas routes), and information about materials used. You should keep this file for the life of the building — it will be needed by any future contractor working on the structure.
As Principal Contractor, TCM takes on the CDM compliance burden so you do not have to. Our project managers are trained in CDM 2015 requirements and manage all documentation as part of the standard project process.
Prepared before work starts on every project, covering site management, welfare, and risk controls
Submitted on all notifiable projects before the construction phase begins
All workers receive a site induction covering the CPP, emergency procedures, and site rules
Compiled throughout the project and handed to you at practical completion
TCM manages all CDM compliance as part of our standard service. Contact us for a free site visit and quote — we will advise on planning, building regulations, and CDM requirements from the outset.