The phrase usually turns up for the first time in a builder’s quote or a letter from building control, and the reaction is always the same: “I thought you were the architect. What is a principal designer, and do I need one?”
It is a fair question. The role has existed for some years, it grew in 2023, and very few homeowners have heard of it before someone asks them to appoint one. This is what it means for a home extension or renovation.
What is a principal designer?
Two different sets of regulations use the same name.
The first is the Construction (Design and Management) Regulations 2015, usually called CDM. They are about health and safety: making sure a project is planned so it can be built, used and maintained safely.
The second arrived in October 2023, when changes to the Building Regulations made under the Building Safety Act added a principal designer role for building regulations compliance. Its job is to plan, manage and monitor the design work so that what gets built complies.
So a typical extension now carries two principal designer duties. They are often held by the same person, and we take both.
Do you need a principal designer for an extension?
On any project where more than one contractor will work, yes. That is nearly every extension once you count the electrician and the plumber. Both sets of rules then require a principal designer and a principal contractor to be appointed.
Where a single contractor does all the work, there is no principal designer, but the designers and the contractor still carry their own duties under both sets of rules.
What does a principal designer do?
For a homeowner, the work comes down to four things.
Coordination. Even a modest extension has an architect, a structural engineer and often a drainage or energy specialist. Someone has to make sure the steel on the engineer’s drawing fits the opening on the architect’s, the drains run where the foundations don’t, and the latest revision is the one being priced.
Competence. Under both sets of rules, everyone doing design work needs the skills, knowledge and experience for what they are designing. The principal designer checks that, and makes sure nobody designs beyond their ability, including a contractor making informal design changes on site.
Health and safety by design. Under CDM, the principal designer looks for risks that can be designed out, such as deep digging next to a neighbour’s foundations, propping during structural openings, or working at height on a roof, and passes information about the remaining risks to the contractor before work starts. The principal contractor then prepares the construction phase plan. At the end, the principal designer prepares the health and safety file, a record of the information you will need for any future work on the house.
Compliance with building regulations. Planning the design work so that what is built meets the regulations, and keeping the information building control needs in order. Planning permission and building regulations explains how the two approvals differ.
What the role is not: it doesn’t supervise the builder day to day, and it doesn’t replace building control inspections.
What happens if you don’t appoint one?
Under CDM, homeowners are treated differently from commercial clients, and this is the part most people find reassuring. A domestic client’s duties pass to the contractor, or to the principal contractor where there is more than one, unless you agree in writing that the principal designer will take them on. If you don’t appoint a principal designer yourself, the designer in control of the pre-construction phase becomes the principal designer by default, and the contractor in control of the build becomes the principal contractor. The building regulations role works in a broadly similar way for homeowners.
The difficulty is that a role filled by default is not the same as a role someone has agreed to and planned for. If the designer who ends up holding it doesn’t know they have it, or their appointment ended at the planning drawings, the coordination simply doesn’t happen. That is where the problems come from: drawings that don’t agree with each other, structural details decided on site, and a completion that drags because information is missing.
This is general guidance rather than legal advice. Who holds which duty on your project should be set out in writing at the start.
When should you appoint a principal designer?
As early as possible, and certainly before the technical design. The role is most useful while the design can still change. Appointed the week before the builder starts, a principal designer can only record risks that are already built into the drawings.
It is also worth checking, when you compare builders, that everyone knows who holds the role. Questions to ask a builder before you sign covers the rest of that conversation.
Can your architect be the principal designer?
Usually, yes, and on a house extension it is often the sensible choice, because the architect is already coordinating the design. We take both principal designer roles. Where we are also supporting the build on site, the role sits within that appointment. Where we are not involved during construction and you need someone to fill the role on its own, we can still take it, and the fee is agreed in writing before work starts. It costs more on its own, because the duties carry on through the build without the regular site contact that makes them manageable.
Like our other later stages, it is set out in the proposal once the Clarity Pack has shown what the project involves. How much an architect costs explains how our fees work, and the process page shows where the role sits among the stages.
If you are not sure what applies to your project, it is a sensible thing to raise on the free 15-minute Clarity Call.
Questions people ask about principal designers
Is a principal designer a legal requirement for a house extension? Yes, where more than one contractor works on the project, which is nearly every extension once the electrician and the plumber are counted. Both the CDM Regulations and the building regulations require one. If you do not appoint one, the designer in control of the design holds the role by default.
Who can be the principal designer? Any designer with the skills, knowledge and experience to plan and coordinate the design work, and the organisation to do it. On a home extension that is usually the architect, because the architect is already coordinating the engineer and the other consultants.
How much does a principal designer cost? When we are already designing and supporting the build, the role sits within those stages. Taken on its own, without the regular site contact that makes it manageable, it costs more, and the fee is agreed in writing before work starts.
