Your team and the process
Who do you need to design and build an extension?
Short answer
Most extensions need five things: someone to design the extension and prepare the drawings, a measured survey of the existing house, a structural engineer for the foundations, beams and new openings, building control from the council or a registered building control approver, and a builder. Where the work affects a shared wall or digs near a neighbour's foundations, the Party Wall etc. Act 1996 requires notice to the neighbour, and party wall surveyors are appointed if a dispute arises.
If more than one contractor will work on the project, which includes a builder who brings in subcontracted trades, you must appoint a principal designer and a principal contractor in writing, under both the CDM Regulations 2015 and the Building Regulations 2010.
The law does not restrict who may prepare the drawings or apply for planning permission. It expects whoever does the design work to be competent to do it.
The roles on a typical extension
| Role | What they do | When you need one |
|---|---|---|
| Designer | Takes your brief, develops options, prepares the planning drawings and the Building Regulations drawings and specification | Always, unless the builder designs it under a design and build arrangement |
| Measured surveyor | Records the existing house accurately, so the design starts from the real building | Almost always; see measured surveys |
| Structural engineer | Designs foundations, beams, lintels and any support for removed walls, with calculations for building control | Whenever the structure changes; see do you need a structural engineer |
| Building control | Checks the design and inspects the work for compliance with the Building Regulations, and issues the completion certificate | Whenever the work is building work under the Regulations; see council building control or a registered approver |
| Party wall surveyors | Agree an award between you and the neighbour under the Party Wall etc. Act 1996 | When notifiable work leads to a dispute, which includes a neighbour not consenting within 14 days to a party structure or excavation notice; see do you need a party wall agreement |
| Principal designer, CDM and Building Regulations | Plan and coordinate the design phase for health and safety and for compliance | When more than one contractor will work on the project |
| Principal contractor | Runs the construction phase | Normally the main builder |
The principal designer appointments
Both sets of regulations use the same trigger: more than one contractor, or more than one reasonably foreseeable. The Building Regulations wording is below; regulation 5(1) of the CDM Regulations 2015 is in almost the same terms.
Where there is more than one contractor, or it is reasonably foreseeable that more than one contractor will be working on a project, the client must appoint in writing (a) a designer with control over the design work as the principal designer for the purposes of these Regulations, and (b) a contractor with control over the building work as the principal contractor for the purposes of these Regulations.
If a homeowner appoints nobody, the Regulations do it for them: the designer in control of the design phase becomes the principal designer and the contractor in control of the construction phase becomes the principal contractor (regulation 11C(2)). The two principal designer roles, and when one person can hold both, are explained in the Building Regulations and CDM principal designer. The CDM side is covered in more depth by ClearCDM's guidance on whether a house extension needs a CDM principal designer.
Who else may be involved
Some sites bring in more people. Each is triggered by something specific.
- A quantity surveyor, where the cost is large or uncertain; see do you need a quantity surveyor.
- A contract administrator, to run the building contract on your behalf; see what a contract administrator does.
- The water company, where you build over or near a public sewer. Requirement H4 of the Building Regulations applies to that work, and regulation 15 requires building control to consult the sewerage undertaker on a full plans application.
- Tree, ecology or heritage specialists, where the council's validation list asks for their reports, for example for trees near the works, protected species or a listed building.
Who may draw it, and the competence the law requires
Government planning guidance says the planning system entitles anyone to apply for permission to develop any plot of land, irrespective of ownership. Nothing in planning law restricts who may prepare the drawings, and the law protects one professional title, not the work. What the Building Regulations do require, since 1 October 2023, is competence. Regulation 11F says anyone carrying out design work must have the skills, knowledge, experience and behaviours to do it so that the building work, if built, would comply. What to look for in whoever designs your project is in anyone can submit a planning application.
Who coordinates whom
The designer and the engineer produce separate information that has to agree: a beam on the engineer's drawing must fit the ceiling height on the designer's section. On most home projects the designer coordinates the engineer's work into the drawings and specification the builder prices. Agree in writing who does that, and who deals with building control, before anyone starts. The order in which the roles come in is shown in the project map.
Sources
- Building Regulations 2010, regulations 11C, 11D and 11F legislation.gov.uk
- Construction (Design and Management) Regulations 2015, regulation 5 legislation.gov.uk
- Building Regulations 2010, regulation 15 and Schedule 1, Part H legislation.gov.uk
- Planning practice guidance: making an application, paragraph 027 gov.uk
- Party Wall etc. Act 1996, section 10 legislation.gov.uk
- ClearCDM: does a house extension need a CDM principal designer? clearcdm.co.uk