Principal Contractor & Principal Designer — Your CDM Duties as a Homeowner

The Construction (Design and Management) Regulations 2015 place legal duties on anyone involved in a construction project — the client (you, the homeowner), the designers (architects and engineers), and the contractors. The aim is to reduce the number of people killed or seriously injured on construction sites.

In 2022, construction accounted for 29% of all worker fatalities in Great Britain (HSE statistics). CDM 2015 is the government's primary regulatory tool to address this.

The 'client' under CDM 2015 is the person or organisation for whom the project is carried out. For a domestic extension or loft conversion on your home, you are the client. Client duties include:

• •Making arrangements for managing the project so it is carried out so far as is reasonably practicable without risks to health or safety
• •Ensuring that the Principal Designer and Principal Contractor carry out their duties
• •Ensuring that a Construction Phase Plan is drawn up before construction starts (for notifiable projects)
• •Ensuring that a Health and Safety File is prepared and handed over at the end of the project

A Principal Designer (PD) is the designer appointed to plan, manage, monitor and coordinate health and safety during the pre-construction phase — i.e., the design stage. Their duties include:

• •Identifying, eliminating and controlling foreseeable risks that arise from the design
• •Ensuring designers comply with their duties
• •Coordinating health and safety matters during the design stage
• •Preparing the Pre-Construction Information Pack and passing it to the Principal Contractor

The Principal Designer is typically the architect or lead designer on the project. They must be appointed in writing by the client before design work begins (for notifiable projects).

A Principal Contractor (PC) is the contractor appointed to plan, manage, monitor and coordinate health and safety during the construction phase. Their duties include:

• •Drawing up and implementing the Construction Phase Plan (CPP)
• •Coordinating health and safety on site across all contractors and trades
• •Ensuring suitable site induction, welfare facilities, and emergency procedures are in place
• •Facilitating cooperation and communication between all contractors
• •Ensuring only authorised personnel are on site

The Principal Contractor must be appointed by the client in writing before the construction phase begins. For domestic projects with more than one contractor, a Principal Contractor must be formally appointed.

RCB Design & Build acts as Principal Contractor on all projects we deliver. This means we take legal CDM responsibility for site safety, not you.

A project is notifiable to the Health and Safety Executive (HSE) if the construction phase is likely to exceed:

• •30 working days and involve more than 20 workers simultaneously at any point, OR
• •500 person-days total

For a standard residential loft conversion or extension (typically 8–16 weeks with 3–8 people on site), most projects will not be notifiable. However, the duties of the Principal Designer and Principal Contractor still apply — notification is a separate threshold from the appointment duties.

Under CDM 2015, if you (the domestic client) do not appoint a Principal Designer or Principal Contractor, you assume all of their duties by default. This means you are personally responsible under law for the CDM duties of those roles.

For most homeowners, this is a significant legal risk. It means that if a worker is injured on your site and no formal PC has been appointed, you may be exposed to enforcement action, prohibition notices, or prosecution by the HSE.

When you appoint RCB as your building contractor in writing, you transfer the Principal Contractor duties to us. We then take on full CDM responsibility for the construction phase.

The Construction Phase Plan (CPP) is a document produced by the Principal Contractor before construction starts. It covers:

• •Description of the project and how it will be managed
• •Site management arrangements (site rules, induction, permits to work)
• •Emergency procedures (fire evacuation, first aid)
• •Specific risks on the project (proximity to neighbours, overhead cables, party wall works)
• •Welfare facilities
• •Communication arrangements between contractors

For smaller residential projects, the CPP is typically a concise document. For larger or more complex projects, it may run to many pages. Building Control may ask to see it.

The Health and Safety File (H&S File) is a document prepared by the Principal Designer and passed to the client at the end of the project. It contains information about the completed project that will be needed for any future work — for example:

• •As-built drawings showing structural elements
• •Location of hidden services (electrical, plumbing, gas)
• •Structural calculations and engineer's certificates
• •Specifications for materials used
• •Warranties and guarantees
• •Party wall awards (if applicable)

The H&S File must be kept by the client and passed on to any future owner of the property. It is increasingly scrutinised by solicitors in property transactions.