Quick Answer
In Berkshire in 2026, a single storey rear extension typically costs £2,000 to £2,800 per m² for a standard specification, rising to £3,200+/m² for a luxury build with Crittall glazing, underfloor heating and bespoke joinery. A common 25–30 m² kitchen-diner extension lands between £52,000 and £90,000 all-in, depending on finish level, structural complexity and groundwork conditions.
Why More Berkshire Homeowners Are Extending Rather Than Moving
Moving house in Berkshire costs far more than most people plan for. Stamp duty, estate agent fees, solicitors, removal vans, and then the inevitable work on the new place — £40,000 to £60,000 spent just to get through someone else's front door. For many families across Reading, Windsor, Maidenhead and Wokingham, a well-designed home extension now delivers more space, more value and far less disruption than uprooting entirely.
We've been building extensions across Berkshire and the Thames Valley since 2014. This guide gives you the real 2026 numbers — not the optimistic estimates you find on calculator sites, but the figures we're actually pricing and building at right now.
Types of House Extension in Berkshire — and What Each Costs
Single Storey Rear Extension
The most common project across Berkshire's Edwardian semis and 1930s detacheds. A single storey rear extension opens up the back of the house — usually creating an open-plan kitchen, dining and living space flooded with natural light via bi-fold doors or a Crittall glazed rear wall.
2026 cost range: £2,000 – £3,200/m² (standard to luxury specification)
A typical 25 m² rear extension comes in at £50,000 – £80,000 all-in, including foundations, structural steels, insulation, electrics, plumbing, plastering and decoration. The kitchen itself is priced separately.
What affects the cost most: depth of the extension (deeper = more steel span), bi-fold or Crittall glazing vs standard windows, underfloor heating, and whether existing drains need rerouting.
Side Return Extension
Particularly popular on Victorian and Edwardian terraces across central Reading, Slough and Windsor where a narrow side passage sits unused alongside the ground floor. Infilling this return with glazed steel dramatically transforms the feel of an otherwise dark and cramped kitchen.
2026 cost range: £2,200 – £3,400/m²
Side returns are smaller in footprint (typically 8–15 m²) but require careful structural work: a steel frame, party wall agreement with the neighbour and often a lantern roof or rooflights to bring in light. Budget £30,000 – £55,000 for a well-executed side return.
Double Storey Extension
Building upwards at the same time as outwards is one of the smartest investments a Berkshire homeowner can make. You get two floors of new space — typically a ground floor kitchen-diner and a first floor bedroom with en-suite — for significantly less per square metre than two separate builds.
2026 cost range: £1,700 – £2,500/m²
Because foundations and roof costs are shared across both floors, the effective cost per square metre drops. A 25 m² footprint double storey (50 m² usable) typically costs £90,000 – £135,000, roughly what you'd pay for two single storey builds.
Planning note: Double storey rear extensions almost always require full planning permission. They sit outside Permitted Development rights. Allow 8–13 weeks for a planning decision from your local authority.
Wraparound (L-Shaped) Extension
A wraparound combines a rear extension with a side return, creating an L-shaped footprint that captures the maximum ground floor space a property can yield. The result — when done well — is a single large, light-filled open-plan zone that transforms the entire feel of the home.
2026 cost range: £2,000 – £3,000/m²
Wraparounds typically range from 35 to 55 m² and cost £75,000 – £165,000 depending on size and specification. They almost always require planning permission and a robust structural design.
Orangery or Garden Room
At the lighter end of the extension spectrum, an orangery or glazed garden room can often be built under Permitted Development, skipping the planning process entirely. Foundations are shallower, structures are lighter, and build times are shorter.
2026 cost range: £1,800 – £2,600/m²
A quality 20 m² garden room with solid roof sections and bi-fold access costs £38,000 – £55,000. These suit homeowners who want a year-round usable space — a home office, a snug, a playroom — without a full structural build.
Berkshire House Extension Cost Table 2026
| Extension Type | Typical Size | Cost Range (All-In) | Build Time |
|---|---|---|---|
| Single Storey Rear | 20–35 m² | £42,000 – £105,000 | 10–16 weeks |
| Side Return | 8–15 m² | £28,000 – £55,000 | 8–12 weeks |
| Double Storey | 40–60 m² usable | £75,000 – £150,000 | 16–24 weeks |
| Wraparound (L-Shape) | 35–55 m² | £75,000 – £165,000 | 16–22 weeks |
| Garden Room / Orangery | 15–25 m² | £30,000 – £60,000 | 6–10 weeks |
Figures cover Berkshire and Thames Valley pricing in 2026. Kitchen, bathroom and furniture are excluded unless stated. VAT at 20% is included.
What Drives Extension Costs Up or Down in Berkshire?
Understanding the cost levers is more useful than any single headline figure. The five biggest variables are:
1. Groundwork and foundations. Clay-heavy soil — common across much of Reading, Wokingham and the Chilterns fringe — can require deeper strip foundations or even pile foundations. A single unexpected ground condition can add £5,000–£15,000 to a job.
2. Structural steels. Every time you remove a wall or create a wide opening, a structural steel beam carries the load. The size and number of steels required depend on your structural engineer's calculations. Larger open spans cost more.
3. Glazing specification. Standard aluminium bi-folds cost a fraction of bespoke Crittall-style steel frames. The jump from a £4,500 bi-fold to a £14,000 Crittall rear wall is one of the most visible ways specification drives up the total.
4. Existing services. If the extension requires relocating drains, gas pipes, electrics or a boiler — and in older Berkshire properties, it often does — you're adding unplanned cost. Always have a builder survey the existing drainage layout before finalising your footprint.
5. Finish level. Marble worktops, bespoke joinery, underfloor heating, smart lighting, frameless glass balustrades — the cost difference between a standard-spec and a luxury finish on the same footprint can easily be £30,000 on a mid-sized project.
Planning Permission for House Extensions in Berkshire
Most single storey rear extensions in Berkshire qualify for Permitted Development (PD), meaning you can build without a full planning application — subject to size limits:
- Terraced and semi-detached houses: rear extensions up to 3 metres deep under standard PD, or up to 6 metres under the Neighbour Consultation Scheme (Prior Approval)
- Detached houses: up to 4 metres standard, or 8 metres under Prior Approval
Permitted Development rights do not apply if:
- The property is in a Conservation Area (common in central Reading, Windsor, Henley-on-Thames and parts of Newbury)
- The property is Listed
- The land is in an Article 4 Direction zone
- You're building a double storey or side extension
When in doubt, a Pre-Application Enquiry with the local planning authority takes 3–4 weeks and gives you a clear steer before you spend money on drawings. We manage this process entirely for SIB clients.
How Much Value Does an Extension Add in the Thames Valley?
Berkshire and the Thames Valley are among the strongest property markets in the UK outside central London. A high-quality kitchen-diner rear extension typically adds 15 to 25% to the value of the property — and sometimes more on under-converted period homes in premium postcodes like Windsor, Henley and parts of Richmond.
A double storey addition that creates an extra bedroom with en-suite pushes that further still. Buyers in Berkshire's commuter belt actively seek four-bedroom homes and pay a meaningful premium over three-bedroom equivalents.
The numbers rarely lie: if your 3-bed Reading semi is worth £480,000 and a well-specified rear extension adds 18% value, you've added £86,000 in equity — at a build cost of £65,000.
The 5 Mistakes Berkshire Homeowners Make When Planning an Extension
1. Getting quotes before having drawings. A quote without architect drawings isn't a quote — it's a guess. Invest in proper architectural drawings first.
2. Choosing the cheapest tender. A quote 30% below everyone else is telling you something. Either scope is missing, materials are being substituted, or there's no contingency for what lies beneath the slab.
3. Underestimating the programme. The typical journey from first conversation to moving your kitchen furniture into the new space takes 9 to 12 months. Anyone promising otherwise is skipping steps.
4. Ignoring party wall obligations. If your extension is within 3 metres of a neighbour's foundation, you need a Party Wall Agreement. It's a legal requirement, and skipping it creates problems during sale surveys later.
5. Separating the extension from the rest of the house. The best extensions feel like they were always there. That requires thinking about the whole ground floor — floor levels, ceiling heights, sightlines, materials — not just the box being added.
How SIB Construction Approaches Extensions
Every extension we build in Berkshire is led personally by Guri from the first site survey to the final handover. There's no account manager layer, no rotating subcontract crew, and no surprises in the invoicing.
Our process for extensions:
- Free site visit — Guri visits, surveys the property, reviews drainage and structure, and understands exactly what you're trying to achieve
- Fixed-price quotation — returned within 48 hours, covering every element of the build scope
- Design & planning — we handle architectural drawings, structural engineering and planning or Prior Approval submissions
- Build — our in-house team, the same faces on site from day one to final snag
- 10-year structural guarantee — on all structural work
We've built extensions across Reading, Windsor, Maidenhead, Slough, Wokingham, Bracknell, Henley-on-Thames and into Surrey and West London.