Why Planning Matters More Than You Think
Most extension problems do not happen on site — they are designed in long before the first brick is laid. A vague brief, an unrealistic budget, missing technical drawings or a rushed builder appointment all create the conditions for delays, overruns and disputes.
The good news is that the planning stages are entirely within your control. Work through them in order, give each one the time it deserves, and you remove most of the risk before you spend the largest part of your money. Here is the full sequence.
The 11 Stages of a House Extension
Define the brief — what problem are you solving?
Before any drawings, write down what the extension needs to achieve. More kitchen space? A bigger living area? A downstairs bedroom or bathroom? A home office? Be specific about how you live and what is not working now. A clear brief is the single biggest driver of a successful project — it keeps every later decision anchored to a purpose rather than to fashion.
Set a realistic budget — including the hidden costs
Decide what you can comfortably spend, then break it down. The build cost is only part of it. Add design fees, structural fees, planning fees, Building Control fees, party wall surveyor costs, VAT, kitchens and bathrooms, finishes, and a contingency of at least 10–15%. Knowing the total envelope early prevents a design you cannot afford to build.
Appoint a designer — architect or architectural technologist
You need someone to turn the brief into drawings. An architect or architectural technologist will produce concept designs, then planning drawings, then technical (construction) drawings. Choose someone whose previous work matches the style and scale you want, and agree their fee stages in writing before you start.
Concept design and feasibility
Your designer produces initial layout options. This is the stage to explore ideas, test what fits, and sense-check against your budget. Expect two or three iterations. A good designer will also flag early constraints — drainage, boundaries, rights of light, trees, and whether permitted development might apply.
Planning permission (or permitted development)
Many single-storey rear extensions fall under permitted development and do not need full planning permission — but size limits, conservation areas and Article 4 directions can change that. If full permission is needed, allow 8–12 weeks for a decision. A Lawful Development Certificate is worth obtaining even under permitted development, as proof for future buyers.
Building Regulations and technical design
Planning permission governs whether you can build; Building Regulations govern how. Your designer and a structural engineer produce construction drawings and calculations covering foundations, structure, insulation, fire safety, ventilation and drainage. These are the drawings a builder actually prices and builds from.
Party Wall agreements (if applicable)
If you are building near or on a shared boundary, the Party Wall etc. Act 1996 may require you to serve notice on neighbours. This can take up to two months, so start it early. Budget for surveyor fees, which you typically pay even for your neighbour's surveyor.
Tender — get like-for-like quotes
With technical drawings and a clear specification, invite two or three contractors to quote. Quoting against the same detailed information is the only way to compare like-for-like. Check insurance, references, and recent comparable projects before you decide — price alone is not enough.
Contract and programme
Appoint your contractor on a written contract that sets out scope, price, payment schedule linked to progress, start date, duration and how variations are handled. A clear contract protects both sides and removes the ambiguity that causes most disputes.
The build — and staying in control
During construction, expect regular Building Control inspections at key stages. Keep a written record of any changes and agree their cost before work proceeds. A weekly catch-up with your contractor keeps small issues from becoming big ones.
Snagging and completion
Near the end, walk the project and produce a snagging list of defects to be put right before final payment. Once snags are resolved and you have your Building Control completion certificate, the project is complete. Keep all certificates and warranties safe — you will need them when you sell.
What Your Budget Actually Has to Cover
The build cost is the headline figure, but it is far from the only one. Budget for all of the following so the true cost does not catch you out:
- Build cost (labour, materials, plant)
- Architect / designer fees (often 7–12% of build cost)
- Structural engineer fees
- Planning application fee
- Building Control fees
- Party Wall surveyor fees (if applicable)
- Kitchen, bathroom and fitted joinery
- Flooring, decoration and finishes
- VAT (20% on most domestic work)
- Contingency (10–15% of total)
Working out this full picture early — before you fall in love with a design — keeps the project anchored to what you can actually afford to build and finish.
Common Planning Mistakes to Avoid
These are the errors we see most often — and each one is avoidable with a little discipline at the planning stage:
Separate Consultants vs Design and Build
The traditional route uses separate parties — an architect to design, an engineer for structure, and a builder to construct. It works, but it leaves gaps: the designer is not responsible for buildability or cost, and the builder is not responsible for the design. When something does not add up on site, each can point to the other.
A design-and-build approach puts design and construction under one accountable team, so buildability and budget are considered from the first sketch and there is a single point of responsibility throughout. For many homeowners, that removes the most stressful part of the whole process.
RCB Can Plan and Build Your Extension
From the first conversation to the completion certificate, RCB manages every stage of your extension under one team — design, planning, Building Regulations, tender and build. One point of contact, one point of responsibility.
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