You have decided to extend. Now you are staring at two different kinds of company, an architect and a builder, and nobody has told you which to call first or what each actually does.
It is the most common confusion at the start of a project, and getting it wrong costs real money. The architects in london homeowners speak to first will usually explain the split before anything else, because the order you hire in shapes the whole build.
Here is who does what, in plain terms, and the order that works.
The Simplest Way to Tell Them Apart
Think of it as design versus construction.
An architect works out what you can build and draws it. They assess your plot, design the extension, handle the planning application, and produce the technical drawings a builder works from.
A builder takes those drawings and physically builds the thing. Groundworks, walls, roof, the lot.
One decides and documents. The other constructs. They are different trades with different training, and most projects need both.
What the Architect Actually Handles
The architect owns the early, invisible part of the project, which is where most of the value and most of the risk sits.
They survey the existing house and establish what the planning rules allow on your specific street. They design the scheme to work for how you live, and crucially to stand a good chance of approval. They prepare and submit the planning application. Then they produce the building regulations drawings and the detailed technical package.
Only someone registered with the Architects Registration Board can legally call themselves an architect, under the Architects Act 1997. That register is public, so you can check it in a minute. Chartered practices, which carry RIBA membership, work to a further professional standard on top.
On a full service, the architect also runs a tender to several builders, helps you pick one, and then monitors the build to make sure it matches the approved design.
What the Builder Actually Handles
The builder turns the drawings into a finished room.
They price the job, order materials, manage the trades, and carry out the construction to the architect’s drawings and the building regulations. A good builder solves the hundred small practical problems that come up on site, and keeps the work to programme and to standard.
When choosing a builder, the checks that matter are concrete. Look for membership of a recognised body such as the Federation of Master Builders, which credit checks and independently inspects its members. Ask for proof of public liability insurance. And ask to see two or three recent, comparable jobs you can actually go and look at or speak to the owners about.
A builder does not need to be ARB registered, because the title is not protected the way architect is. That is exactly why the vetting falls to you, or to the architect acting on your behalf.
So Who Do You Hire First?
Almost always, the architect comes first. Here is why the order matters so much.
If you hire a builder before there is a design, you are asking them to price something that does not exist yet. You get a vague number that moves the moment real drawings appear. Worse, anything not nailed down in drawings becomes a variation later, billed when the work is half done and you have no leverage.
Hire the architect first and the sequence protects you. The design is fixed, the planning is secured, the technical drawings are complete, and then several builders price the same clear, detailed package. You are comparing like for like, and the quotes are real.
There is one exception worth knowing. On a design and build route, a single firm handles both the design and the construction under one contract, so the question of order disappears. That suits straightforward projects where cost certainty and a single point of contact matter more than independent oversight.
How the Two Work Together on Site
The relationship between architect and builder is where a project runs smoothly or turns into a fight, and the structure you set up at the start decides which.
On the traditional route, the architect can administer the building contract. That means someone who works for you, not the builder, inspects the work as it goes and signs off payments against genuine progress. It is a quiet but powerful protection, because it puts an expert eye between your money and the build.
Where this breaks down is when there is no clear contract and no one coordinating. The architect blames the builder, the builder blames the drawings, and you are stuck in the middle paying for the gap. A proper contract, a recognised form such as a JCT agreement, and a clear line of responsibility prevent most of that before it starts.
This is also where searching for trustworthy builders near me pays off, because a local builder with a real, checkable track record and a willingness to work under a proper contract is worth far more than the cheapest quote with none of that behind it.
The Honest Summary
For most London extensions, the answer is clear. Hire the architect first, let them design the scheme and secure the planning, then tender the finished drawings to vetted builders and appoint one under a proper contract, ideally with the architect monitoring the build.
Extension Architecture works this way across London and Surrey, keeping architects, planning consultants and structural engineers in one team, which is part of why a scheme like the South Croydon project, refused under a previous design and then redesigned and approved, held together: one accountable team shaping the design, the planning and the build sequence rather than three parties each owning a piece. Get the order right, and the two trades stop being a source of confusion and start being what they should be, a design you can trust and a build that delivers it.
