Step 1: Work out what you want and what it might cost
Before spending money on architects or planning applications, be clear on what you are trying to achieve. Ask yourself:
- What space do I actually need? (Larger kitchen, extra bedroom, home office, utility room?)
- How big does the extension need to be?
- What is my rough budget, including a 15–20% contingency?
- Am I prepared for the disruption? (Typically 3–6 months of construction on site)
Use our cost guides as a starting point: a single-storey rear extension costs £1,500–£2,800 per m² across the UK before VAT and professional fees. See our full house extension cost guide for a detailed breakdown.
Step 2: Check your Permitted Development rights
Before appointing an architect, find out whether your extension needs planning permission. Many single-storey rear extensions fall within Permitted Development (PD) rights — meaning no formal application is needed.
The quick check: call your local council's planning duty officer. It takes 10 minutes and is free. They will tell you definitively whether your proposed extension needs consent.
See our full guide: Planning permission for a house extension.
Step 3: Appoint an architect or designer
Even if planning permission is not required, you need drawn plans for Building Regulations and for your contractor to build from. An architect, architectural technologist or architectural designer can all produce these drawings — the difference is mainly in qualifications and fee level.
For a simple single-storey extension, expect to pay:
- Planning drawings only: £1,500–£4,000
- Planning + Building Regulations drawings: £3,000–£7,000
- Full service (design through to construction monitoring): 8–12% of build cost
Get two or three quotes. Ask to see examples of similar projects they have completed and check that they have professional indemnity insurance.
Step 4: Appoint a structural engineer
A structural engineer is required on almost every extension. They design:
- The foundation type and depth (based on ground conditions)
- Any steel beams needed to span openings (e.g. where the existing wall is opened up)
- Wall tie specifications and roof structure loadings
Your architect can usually recommend a structural engineer, or you can find one independently. Budget £800–£2,500 for a typical single-storey extension. The structural engineer's drawings are required as part of the Building Regulations submission.
Step 5: Apply for planning permission (if needed)
If your extension is not within Permitted Development, submit a Householder Planning Application through the Planning Portal (planningportal.co.uk). The fee is £258 in England (2026). The council has 8 weeks to decide.
If it is within PD but you want formal confirmation, apply for a Certificate of Lawful Development (~£129). This protects you when you sell the property later.
Step 6: Submit Building Regulations drawings
Building Regulations approval is separate from planning permission and is required for almost all extensions. Your architect and structural engineer's drawings are submitted to either the local council's Building Control department or a private Approved Inspector.
There are two routes:
- Full Plans application: drawings submitted and checked before work starts. Takes 5–8 weeks. The safer option — any issues are flagged before building begins.
- Building Notice: no drawings submitted upfront — the inspector checks the work on site at each stage. Faster to start, but riskier if there are problems.
Most architects recommend the Full Plans route for extensions.
Step 7: Serve Party Wall notices (if required)
If your extension involves excavating within 3 metres of your neighbour's foundations, or building on or close to a shared boundary, you must serve written notice on affected neighbours under the Party Wall etc. Act 1996. This must be done before work starts — at least 2 months for party wall work, 1 month for adjacent excavations.
Neighbours can consent in writing or appoint a party wall surveyor. If they appoint a surveyor, you pay their fees.
Step 8: Get contractor quotes
Once you have planning permission (if needed) and Building Regulations drawings, you are ready to get contractor quotes. Send the same drawings to at least three contractors so you are comparing like for like.
A good quote should be itemised: foundations, brickwork, roof, windows, first fix, plastering, and any specialist items. Be wary of very low quotes — they usually indicate something has been missed or will be added later.
For advice on choosing the right contractor, see our guide to choosing a bricklayer — the same principles apply to any building contractor.
Step 9: Construction — what to expect on site
The typical build sequence for a single-storey rear extension:
- Strip out: existing patio or path removed; any existing structure demolished
- Foundations: excavated and poured (allow 5–7 days to cure before bricklaying)
- Brickwork and blockwork: cavity walls built up to roof level, with wall ties, insulation and damp proof course installed as the work progresses (typically 2–4 weeks)
- Roof structure: roof joists or trusses installed and made weathertight
- Structural opening: wall between extension and house opened up, steel beam installed (one of the most disruptive stages — usually 2–4 days)
- First fix: plumbing, electrics and heating routes installed in walls and floor
- Plastering and screed
- Second fix and fit-out: windows, doors, kitchen, flooring, decoration
Building Regulation inspection visits happen at key stages: foundations (before concrete is poured), damp proof course level, structural steel, roof, and final completion sign-off.
Step 10: Get your completion certificate
Once the build is finished and all inspections have passed, your Building Control inspector issues a completion certificate. Keep this document safely — you will need it when you sell the property. Without it, the sale can be delayed or fall through.
Do not pay the final payment to your contractor until you have the completion certificate and are satisfied with the work. Retaining 2.5–5% until completion is standard practice and protects both parties.
Frequently asked questions
- How long does the whole process take from start to finish?
- From first thinking about an extension to moving back into the completed room: typically 9–18 months. Planning drawings take 4–8 weeks. Planning permission (if needed) takes 8 weeks. Finding a contractor and mobilising takes 4–12 weeks. Construction takes 10–20 weeks for a single-storey extension. Fit-out takes 4–8 weeks. Getting the sequence right from the start avoids delays.
- Can I use different contractors for different parts?
- Yes — many homeowners use a specialist bricklayer for the structural shell, a separate roofer, and their own kitchen fitter and decorator for the fit-out. This can save money compared to using a single main contractor who adds margin on all subcontracted trades. However, it requires more coordination from you as the client.
- What contingency should I allow?
- A contingency of 15–20% of the build cost is standard advice for extension projects. Unexpected costs commonly arise from ground conditions (foundation depth greater than expected), discovery of asbestos or buried services, and changes to specification mid-build. Having the contingency in place avoids difficult decisions if something unexpected arises.