Planning a Home Extension in the UK: A Contractor's Step-by-Step Guide
Most extension projects that go wrong do so before the first spade goes in. Here is what you need to know before you start, and the things that catch people out most often.
Before You Do Anything Else
The most expensive mistake you can make when planning an extension is starting with the contractor. Start with the constraints. Talk to your local planning authority (or use the Planning Portal) to understand what you're allowed to build before you commission a single drawing. Talk to a structural engineer if you're removing or altering any walls. Commission an asbestos survey if the property was built before 2000, particularly relevant for any work to existing roofs, floors, or internal partitions. The information from these early conversations shapes everything that follows. It can save you from designing something you cannot build, or building something that needs pulling down.
Step 1: Establish Feasibility
Planning — What Can You Build?
In England, many single-storey rear extensions fall within permitted development (PD), meaning no full planning application is required. The rules allow up to 4 metres deep on a detached house or 3 metres deep on a semi-detached or terraced property, provided the extension doesn't exceed the original roof height, doesn't cover more than 50% of the garden, and meets a range of conditions relating to materials, windows, and boundary proximity.
Permitted development rights do not apply to listed buildings (which always need listed building consent), properties in conservation areas (stricter rules apply), flats and maisonettes (no PD rights at all), or properties where PD rights have been removed by a planning condition. This last one catches people regularly, particularly on some 1980s and 1990s estates. Always check before assuming PD applies.
If your extension exceeds the PD limits, a deeper rear extension or anything double-storey, you need a formal householder planning application. That means submitting drawings, paying the fee (£258 for a householder application in England as of 2026), and waiting. The statutory determination period is 8 weeks, though many councils take 10–14 weeks in practice.
Side extensions are a particular watch-out. They are often not permitted development and planners are generally protective of the gap between houses and the street scene. If you're thinking about a side extension, check the rules for your specific property before commissioning detailed drawings.
Party Wall Act
If your extension is within 3 metres of a neighbouring property's foundation, or involves excavation within 6 metres of a neighbouring structure, the Party Wall etc. Act 1996 applies. You must serve notice on affected neighbours before work begins. If they agree, you can proceed. If they dissent or fail to respond, you must appoint a party wall surveyor. You pay their costs as well as your own.
Serve notice early. It must go out at least 2 months before the proposed start date for most party wall works. Leaving it until you're ready to start creates a delay you cannot control. Most neighbours who understand the process are happy to agree, particularly if you talk to them before the formal notice arrives rather than letting a legal document be their first notification.
Budget Feasibility
Before commissioning detailed design, run a rough cost check to confirm the project is financially viable. Use the build cost calculator to get a per-m² estimate for your specification level and region. Add 10–15% for professional fees, 10% contingency, any utility connections, and VAT (20% on most extension work). If the total exceeds what the project adds to the property's value, the economics need revisiting before anyone picks up a pencil.
Step 2: Design and Approvals
Commissioning a Designer
For a straightforward single-storey extension, a competent architectural technician or experienced designer will produce what you need at lower cost than a fully qualified architect. For anything involving structural changes, listed buildings, or conservation areas, a chartered architect (RIBA) with local planning experience is worth the additional fee. Extension design fees typically run £1,500–5,000 for drawings and specification production, and more if the architect manages the project as contract administrator.
Check that your designer has professional indemnity (PI) insurance. This protects you if an error in the design causes a problem during construction or at sale. Ask to see the certificate before appointing anyone. A designer who resists this question is a red flag.
Building Regulations
Building regulations approval is separate from planning permission and applies to almost all extensions. You have two routes: Full Plans (submit detailed drawings before starting, get approval, inspections at key stages) or Building Notice (notify the council you're starting, inspector visits during construction without pre-approved drawings). Full Plans is the better choice for anything above a very simple extension. It confirms design compliance before you spend money building something that might need changing.
Building regulations cover structural stability, fire safety, thermal performance under Part L, drainage connections, and ventilation. Your structural engineer and designer handle these through their calculations and drawings. The building inspector will want to see the evidence at key stages. Don't let your contractor skip the inspections to save time.
Structural Engineer
Any extension that involves removing or altering a structural wall, or creating a new opening in an existing wall, requires a structural engineer's design. That means calculations for the steel beam or concrete lintel spanning the opening, the padstone bearings at each end, and any effect on the foundation below. Structural engineer fees for residential extension work typically run £800–2,500 depending on complexity. Their calculations form part of the building regulations submission.
Do not skip the structural engineer to save a few hundred pounds. An incorrectly sized beam over a new opening causes cracking, deflection, and in the worst cases structural failure. The building inspector requires the calculations before signing off. There is no shortcut here.
Step 3: Finding and Appointing a Contractor
How Many Quotes?
Get at minimum three quotes from contractors who have been recommended or have verifiable references on similar work. Do not select on price alone. The cheapest quote on an extension is very rarely the best value. A contractor who comes in significantly below the other two has either made an error, plans to recover the margin through variations, or does not understand what the job involves.
Ask for references from clients on similar work in the past 18 months and actually call them. Ask: did the project finish on time and budget? Would you use them again? Were problems handled professionally? A contractor with a string of satisfied clients who will talk freely is a good sign. A contractor who cannot produce references on similar recent work is a risk, full stop.
Checking a Contractor's Background
Check Companies House to confirm the company exists and the director has not been disqualified. Check employers' liability insurance (legally required if they employ anyone) and public liability insurance (minimum £2m, more for larger projects). For gas work, check Gas Safe registration. For electrical work, check NICEIC, NAPIT, or ECA registration. Registered electricians can self-certify, which saves you the cost of a separate inspection.
Be cautious of contractors asking for large upfront payments. A reasonable deposit on a residential extension is 5–10% on signing, with staged payments tied to measurable milestones. Asking for 30–50% before work starts is unusual and a risk you don't need to take.
The Contract
Use a written contract. The JCT Minor Works Building Contract is the industry standard for residential extensions and small building works. It covers scope, price, programme, insurance, variations, payment terms, and dispute resolution. It is not long. It is not complicated. A letter or email exchange is not adequate for work over £10,000, and I say that from experience of seeing what happens when things go wrong without one.
The contract should identify the scope of work (with reference to the drawings and specification), the contract sum, the programme (start date and anticipated completion), the payment terms, who carries insurance on the works and the existing structure, and how variations will be priced and approved.
Step 4: Construction and What to Expect
Typical Programme for a Single-Storey Extension
A well-run single-storey rear extension of 20–30m² typically takes 10–16 weeks on site from groundbreaking to practical completion. That assumes no significant unforeseen conditions and timely decision-making by the client. The stages and approximate durations:
Groundworks and foundations: 2–3 weeks. Excavation, concrete strip or raft foundation, DPC level, slab preparation. This is where hidden drainage, made-up ground, and tree roots get discovered. It is the most common source of unforeseen cost on residential extensions.
Superstructure: 3–5 weeks. Brickwork and blockwork to wall plate, cavity insulation, lintels over openings, wall ties. The pace here depends heavily on the bricklayer's productivity and the complexity of the bond specified.
Roof: 1–2 weeks. Rafters or trusses, sarking, battens, tiles, leadwork to junctions. A flat roof in EPDM or GRP typically takes 2–3 days once the structure is in place.
First fix: 2–3 weeks. Structural opening in the existing wall (beam installation), first-fix plumbing and electrics, floor insulation and screed, window and door installation. The beam installation is the most disruptive phase. It typically requires the property to be vacated for 1–3 days.
Plastering and second fix: 2–3 weeks. Plasterboard, skim or wet plaster, second-fix plumbing and electrics, kitchen or bathroom fit-out, decoration. Plaster takes 4–6 weeks to fully dry. That drying time is often the bottleneck before floor finishes can be installed, regardless of how fast everything else runs.
Living Through a Build
If you're living in the property during construction, prepare for real disruption. Particularly during the structural opening phase. Dust and vibration travel through the entire building. The kitchen is typically out of use for at least a few weeks. Garden access may be restricted throughout. Discuss the sequence and the worst disruption points with the contractor at the pre-start meeting. Better to plan around them than be caught out mid-build.
Step 5: Snagging and Handover
What is a Snagging List?
A snag is a defect or incomplete item that needs remedying before the project is considered complete. At practical completion, walk the finished work with the contractor and produce a snagging list of anything not to contract standard. Common snags: minor cosmetic plaster imperfections, paint finishes that need a second coat, doors and windows that need adjustment, tile grout that needs re-pointing, functionally incomplete items. Write them all down on the day. Memory fades and contractors move on to the next job fast.
Retain 2.5% of the contract sum at practical completion until the snags are rectified. The full retention (5%) is held from interim payments throughout the contract, with half released at practical completion and the balance at the end of the defects liability period (typically 12 months). Without retention, a contractor who has been fully paid has much less reason to come back for a half-day of snagging work.
Building Regulations Sign-Off
At the end of the project, the building control inspector carries out a final inspection and issues a Completion Certificate. This confirms the work meets building regulations. Keep it in your property file. Solicitors will ask for it when you sell. If a completion certificate was not obtained for previous work (extremely common with older extensions) it causes problems at sale and may require a retrospective inspection or indemnity insurance to resolve.