Planning a Home Extension in the UK: A Contractor's Step-by-Step Guide
From the initial feasibility check to the moment you get the keys to your finished extension — what to do, what to watch out for, and how to avoid the most common mistakes.
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 any drawings. 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 comes after and can save you from designing something you can't build, or building something that needs to be pulled down.
Step 1: Establish Feasibility
Planning — What Can You Build?
In England, many single-storey rear extensions fall within permitted development (PD) — meaning you don't need full planning permission. The rules allow a single-storey rear extension of up to 4 metres deep on a detached house, or 3 metres deep on a semi-detached or terraced property, provided the extension does not exceed the original roof height, does not occupy more than 50% of the garden, and meets a range of other conditions relating to materials, windows, and proximity to boundaries.
However, permitted development rights do not apply to: listed buildings (always need listed building consent), properties in conservation areas (some extensions still allowed, but stricter rules apply), flats and maisonettes (no PD rights at all), and some properties where PD rights have been removed by a condition on the original planning consent — common on some 1980s and 1990s estates. Always check before assuming PD applies.
If your extension exceeds the PD limits — for example, a rear extension deeper than 4m, or a double-storey extension — you will need a formal householder planning application. This involves submitting drawings, paying a fee (currently £258 for a householder application in England as of 2026), and waiting for a decision. The statutory determination period is 8 weeks, though many councils take longer in practice.
Side extensions in particular are often not permitted development and require full planning permission — planners are generally protective of the gap between houses and the street scene. If you're considering a side extension, check the rules for your specific property and area 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 the affected neighbours before work begins. If they agree to the works, you can proceed. If they dissent or fail to respond, you must appoint a party wall surveyor — and you pay their costs as well as your own.
Serving notice early is in your interest. Notice must be served 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 can't control. Most neighbours who are served notice and understand the process are happy to agree — particularly if you engage with them early and explain what you're proposing.
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, utility connections if applicable, and VAT (standard rate at 20% for most extension work). If the total comes to more than the project adds to the value of your property, the economics need revisiting before you invest in detailed design.
Step 2: Design and Approvals
Commissioning a Designer
For a straightforward single-storey extension, a competent architectural technician or experienced designer will typically produce the drawings you need at lower cost than a fully qualified architect. For more complex work — particularly anything involving structural changes, listed buildings, or conservation areas — a chartered architect (RIBA) with local planning experience is worth the additional fee. Fees for extension design typically run £1,500–5,000 for drawing and specification production, and more if the architect manages the project as contract administrator.
Always check that your designer has professional indemnity insurance (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.
Building Regulations
Building regulations approval is separate from planning permission and applies to almost all extensions. You have two routes: Full Plans (you submit detailed drawings before starting, get approval, and inspections are carried out at key stages) or Building Notice (you notify the council you're starting, and the inspector visits during construction without pre-approved drawings). Full Plans is better for anything above a very simple extension — it confirms design compliance before you spend money on construction.
Building regulations cover: structural stability (the foundations, walls, and roof must be designed to carry the loads imposed), fire safety (escape routes, smoke alarms, fire resistance of elements), thermal performance (insulation levels under Part L), drainage (connection to existing foul and surface drainage systems), and ventilation (extract ventilation in kitchens and bathrooms, background ventilation in habitable rooms). Your structural engineer and designer will typically address these through their calculations and drawings.
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. This means calculations for the steel beam or concrete lintel spanning the new 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.
Don't skip the structural engineer to save money. An incorrectly sized beam over a new opening can cause cracking, deflection, and — in the worst cases — structural failure. The building inspector will typically require the calculations before signing off.
Step 3: Finding and Appointing a Contractor
How Many Quotes?
For a residential extension, get at minimum three quotes from contractors who have been recommended or have verifiable references on similar work. Do not select purely on price — the cheapest quote on an extension is very rarely the best value. A contractor who submits a number significantly below the other two has either made an error, is planning to recover their margin through variations, or does not understand what is involved.
Ask for references from clients on similar work in the past 18 months and follow up on them. Ask specifically: did the project finish on time and budget? Would you use them again? Were any problems handled professionally? A contractor with a string of satisfied clients who will talk freely is a strong signal. A contractor who cannot provide references on similar recent work is a risk.
Checking a Contractor's Background
Check Companies House to confirm the company exists and that the director has not been disqualified. Check that they carry employers' liability insurance (legally required if they employ anyone) and public liability insurance (minimum £2m, more for larger projects). For work that involves gas, check Gas Safe registration. For electrical work, check NICEIC, NAPIT, or ECA registration — registered electricians can self-certify their own work, which saves you the cost of a separate inspection.
Be cautious of contractors who ask for large upfront payments. A reasonable deposit on a residential extension contract is 5–10% on signing, with staged payments tied to completion of measurable milestones. Asking for 30–50% before work starts is unusual and risky.
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 or complex but it is specific enough to protect both parties. A letter or email exchange is not adequate for work over £10,000.
The contract should identify: the scope of work (with reference to the drawings and specification), the contract sum, the programme (start date, anticipated completion date), the payment terms, who is responsible for insurance of the works and existing structure, and how variations will be priced and agreed.
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, assuming 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, or tree roots are most likely to be discovered — the most common source of unforeseen cost.
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 brick bond specified.
Roof: 1–2 weeks. Rafters or trusses, sarking, battens, tiles, leadwork to junctions. A flat roof (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 for the occupants — 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, decorating. The drying time for plaster (typically 4–6 weeks to fully dry) is often the bottleneck before floor finishes can be installed.
Living Through a Build
If you're living in the property during construction, prepare for significant 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. Access to the garden may be restricted throughout. Discuss the sequence and disruption points with the contractor at the pre-start meeting so you can plan around them.
Step 5: Snagging and Handover
What is a Snagging List?
A snag is a defect or incomplete item that needs to be remedied before the project is considered complete. At practical completion, the contractor should walk the finished work with you and you should produce a snagging list of anything that is not to the contract standard. Common snags include: minor cosmetic plaster imperfections, paint finishes that need a second coat, doors and windows that need adjustment, tile grout that needs pointing, and items that are functionally incomplete.
Retain a percentage of the contract sum at practical completion (typically 2.5%) until the snags are rectified. The full retention (typically 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). This provides financial incentive for the contractor to rectify defects promptly.
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 is the formal document confirming that the work meets building regulations requirements. Keep this in your property file — it will be required by solicitors when you sell the property. If a completion certificate was not obtained for previous work (a very common situation with older extensions), it can cause problems at sale and may require a retrospective inspection or indemnity insurance.