Once you have chosen the design and build route, the harder question arrives: which company, and on what terms. The gap between design and build companies london wide is not mostly about price. It is about what sits in the contract, who carries the risk, and what happens when something on site does not go to plan.
Here is what separates a company worth signing with from one worth walking away from, in the order it actually matters.
The Contract Is the Whole Game
A quote is not a contract. A signed, accepted quote can technically form one, but it leaves the terms that cause disputes undefined: payment stages, variations, delays, defects. For anything beyond a small repair, you want a proper written contract on top of the quote.
Two standard forms exist specifically for homeowners, and they are inexpensive. The JCT Home Owner Contract and the FMB Domestic Building Contract. Both are designed for exactly this situation and both are weighted fairly rather than drafted to protect the builder alone.
A company that will only work on its own paperwork, or on a bare quote with no contract, is asking you to accept unnecessary risk. That is the first thing to check, and the easiest.
At minimum the contract should name the exact legal names of both parties, define the scope with drawings and specification referenced, state a fixed price or a transparent pricing basis, set a stage linked payment schedule, give start and completion dates, include a written variations procedure, and cover insurance, warranty, retention and dispute resolution.
If any of those are missing, they get filled in later by whoever has the leverage, and during a build that is rarely you.
Green Flags Worth Looking For
Some signs tell you a company runs projects properly.
A deposit of 5 to 10 per cent, or nothing beyond materials on order. Established firms often ask for very little up front because they are not funding the job from your money.
Stage payments tied to real milestones, ideally to Building Control inspections rather than to dates. Payment on the foundation sign off, the frame, the roof. That structure protects you, because you are paying for work that has been done and checked.
A willingness to take the deposit on a credit card, which gives you Section 75 protection. A retention of around 5 per cent held back until snagging is complete. And a written variations procedure, so any change to the brief gets priced and agreed in writing before it happens rather than appearing on the final invoice.
Red Flags Worth Walking Away From
Others tell you to stop.
A demand for 25 per cent or more up front, or any request for full payment before completion, is a serious warning. It usually means the company needs your money to function, which is not a company you want holding your project.
Cash only, or reluctance to put anything in writing. A deposit with no protection and no explanation of where it sits. Vague scope that leaves the specification open. And pressure to sign quickly, which is the oldest tactic there is.
None of these on their own proves bad faith. Together, or combined with a reluctance to use a standard contract, they are a pattern.
The Question People Forget
Design and build carries an insurance wrinkle the traditional route does not, and it is worth understanding.
Because the same company designs and builds, the contractor takes on design responsibility. Under the JCT Design and Build Contract, that pulls professional indemnity cover into scope, on top of the usual public liability and contract works insurance. Ask for evidence of all three.
There is a second point specific to extensions. Where work is done to an existing house you own, someone has to insure the existing structure against damage caused by the works. Standard JCT wording can leave this with you, on your own buildings policy, with the contractor liable only for their own negligence. It is worth telling your buildings insurer and your mortgage lender that work is starting, because many homeowners overlook this and find a gap exactly when they need cover.
Credentials That Actually Mean Something
Titles are inconsistent in this trade, so check the ones that carry weight.
If the company does its own design, ask who the architect is and for their ARB registration number. Only ARB registered professionals may use the title architect, and the register is public.
For the building side, membership of the Federation of Master Builders means credit checked, insured and subject to independent inspection. It is not a guarantee, but it is a filter.
And ask specifically about structural work. Building Control needs a calculation pack covering beams, foundations and connections before signing off structural work, and that requires a chartered structural engineer. Whether the company has one in house or subcontracts it changes both the coordination and the timeline.
This is one place the structure of the company genuinely matters. Most practices outsource structural engineering, which introduces a handoff every time the design and the calculations need to agree. Extension Architecture keeps a structural engineer within the same team as the design staff, so beam spans get tested while the layout is still being worked out rather than after it is fixed. On a design and build project, where design and construction sit under one roof anyway, removing that extra handoff is where the route earns its keep.
Where Local Knowledge Changes the Job
The right company for a project also understands the ground it is building on, and in parts of Surrey that is not a figure of speech.
Weybridge and the surrounding Elmbridge area sit largely within the Green Belt, and much of the local stock is on London clay, where foundation depths run deeper than the standard metre and mature trees change the calculation again. A company that knows the local planning position and the local ground conditions prices a job accurately. One that does not discovers both during the build, as variations.
That local knowledge is also a planning asset. Extension Architecture handled a Weybridge project sitting in both a flood zone and a conservation area, and by establishing both constraints up front and answering them in the submission, the scheme was approved in around seven weeks. Anyone comparing architects weybridge homeowners might appoint should ask directly about experience with the local council and the local ground, because both show up in the final cost.
The Short Version
Insist on a recognised contract. Treat a large up front deposit as a warning. Tie payments to inspections. Check ARB registration, FMB membership and structural engineering arrangements. Confirm all three insurances, and tell your own insurer work is starting.
Do those, and you have filtered out most of the companies that cause the horror stories, before signing anything.
