Choosing the wrong contractor for a home extension in London is a costly mistake. Not just financially — though that's real — but in time, disruption, and the stress of managing a project that isn't being run properly. This guide is for homeowners who are approaching the market and want to know what a rigorous selection process looks like, what questions actually reveal contractor quality, and what a proper pre-construction process should include.
Planning Permission or Permitted Development — Know Before You Brief Anyone
Before you approach contractors, you need to understand whether your extension requires planning permission or falls under Permitted Development (PD). This affects your timeline by 8–12 weeks and your design freedom considerably.
Permitted Development rights allow most single-storey rear extensions up to 4m (detached) or 3m (semi/terraced) without planning permission, subject to height limits and other conditions. The prior approval process under the "Householder Prior Approval" route can extend this to 6m (semi/terraced) or 8m (detached) under the Larger Home Extension scheme.
Permitted Development does not apply if:
- The property is in a Conservation Area, National Park, AONB, or World Heritage Site (restrictions apply to side and rear extensions)
- The property has Article 4 Directions removing PD rights (common in some London boroughs)
- The property is a flat or maisonette (PD does not apply)
- Previous extensions have already used up the allowable volume
A contractor who tells you definitively that your project is PD without checking your property and borough is guessing. A good contractor will check before committing to a timeline.
Questions That Reveal Contractor Quality
These aren't tick-box questions. They're designed to surface how a contractor actually works, not how they present themselves.
"Who is your structural engineer, and when do they get involved?"
On any extension involving structural steelwork — which includes virtually all extensions with wide openings or removed rear walls — a structural engineer is required. A contractor who handles this in-house or has an established working relationship with a named engineer is set up properly. A contractor who says "we'll get an engineer if we need one" is not.
"How do you handle Building Control?"
They should notify Building Control (or appoint an Approved Inspector), manage staged inspections, and obtain the completion certificate at the end. That certificate is important — mortgage lenders and solicitors ask for it. A contractor who leaves Building Control to you, or who is vague about the process, is shifting risk onto you.
"Can I see examples of completed projects at a similar scale?"
Photographs of finished kitchens are not evidence of construction quality. Ask to see photographs of the structure during construction — steel installation, roof build-up, insulation, drainage — and ideally ask to speak to a previous client. A confident contractor will facilitate this without hesitation.
"What does your contract look like?"
A fixed-price, itemised contract with a defined payment schedule tied to construction stages is the standard for a reputable contractor. Payment schedules that front-load deposits (more than 10–15% upfront) are a warning sign. An absence of a written contract is grounds for walking away.
"Who is on site day to day?"
On a well-run extension project, there is a named site manager or foreman who is present throughout. If the person you're meeting with is also going to be the only person on site, ask how they manage when they're quoting other jobs or off sick.
"What accreditations do you hold?"
FMB (Federation of Master Builders), TrustMark (government-endorsed quality scheme), Checkatrade, and TrustATrader all require contractors to be vetted. They are not a guarantee of quality, but they represent a meaningful baseline — particularly TrustMark, which is underpinned by technical assessment.
Red Flags to Take Seriously
No written contract. Non-negotiable. If a contractor proposes to work on a handshake or a brief email, that arrangement protects neither party.
Large upfront deposit. A deposit of 10–15% is reasonable to cover materials ordering. Requests for 30–50% upfront before work starts are a serious warning sign, particularly from contractors you've just met.
No fixed price. Some variation clauses are legitimate. A contract that is entirely day-rate or "approximate" gives you no protection against cost escalation.
Pressure to start immediately. A busy, reputable contractor has a programme. Pressure to sign and start this week — particularly to "lock in a price" — is a sales tactic, not a logistical necessity.
Vagueness about planning and Building Control. These aren't optional regulatory formalities. A contractor who doesn't bring them up unprompted, or who suggests they're not needed, may be trying to avoid the scrutiny that formal approval involves.
No insurance documentation. Public liability insurance (minimum £2m, ideally £5m) and employers' liability insurance are legal requirements. Ask for the certificates before work begins.
What a Proper Pre-Construction Process Looks Like
A serious contractor does not give you a price after a 20-minute site visit. A proper pre-construction process for a home extension involves:
- Initial site assessment — understanding the property, the brief, any structural implications, and the planning position
- Design development — if design-and-build, this happens in-house; otherwise, coordination with your architect's drawings
- Structural coordination — engaging the structural engineer early, before pricing, so that steelwork is properly specified
- Detailed pricing — itemised, with material allowances clearly stated
- Pre-start meeting — confirming programme, access arrangements, welfare provisions, neighbour notifications, and Building Control notification
- Contract execution — before any work begins
Skipping steps in this sequence to get to site faster is where corners get cut and problems originate.
RCB's Approach to Home Extension Projects
RCB Design & Build are A2Z Principal Contractors covering Greater London and surrounding counties. Extensions — rear, side, double-storey, and wraparound — are central to what we do. We manage the full project: design coordination, structural engineering, planning and PD advice, Building Control, construction, and fit-out, on a single fixed-price contract.
We hold FMB, TrustMark, Checkatrade, and TrustATrader accreditations, and we work to a transparent pre-construction process before any contract is signed.
If you're in the early stages of planning an extension and you want a straightforward conversation about what the project involves, what it's likely to cost, and how it should be run — get in touch.
Contact us:
- Email: contact@rcbgroup.co.uk
- Phone: +44 7359 872594
- Web: [rcbgroup.co.uk](https://rcbgroup.co.uk)