It is the question every homeowner wants answered before anything else: how much is this going to cost? And it is also the hardest to answer in a single figure, because no two extensions are the same. The price depends on the size, the specification, the ground you are building on and the finish you choose. What we can do is give you a realistic sense of the ranges involved in 2026, explain what actually drives the numbers, and help you budget properly so there are no nasty surprises halfway through.
A quick but important caveat before we start: every figure below is an indicative guide for 2026, intended to help you plan. The South East, including Horsham and the wider West Sussex area, typically sits above the national average for building costs, and the only way to get an accurate number for your home is a proper site visit and an itemised quote. Treat this as a way to set your expectations, not a fixed price list.
Indicative extension costs in 2026
As a rough guide, a good-quality single storey extension in the Horsham area in 2026 tends to work out somewhere in the region of £2,500 to £3,500 per square metre for the building work, before you add fitted elements like a kitchen. That means a modest single storey rear extension of around 15 to 20 square metres often lands somewhere between roughly £40,000 and £70,000 once you account for a reasonable specification, though smaller or simpler projects can come in below that and high-end finishes well above it.
Double storey extensions are more cost-effective per square metre than they first appear, because the foundations and roof are shared across two floors. So while the total cost is higher, the cost per square metre of new space is often lower than a single storey project. A double storey extension adding a ground floor room plus a bedroom and bathroom above will typically start in the region of £70,000 and rise from there depending on size and finish. Again, these are ballpark figures for planning purposes rather than a quote.
What a proper quote should include
One of the biggest reasons homeowners feel stung by home extension costs is a quote that was never complete in the first place. A low headline number that leaves out half the job is not a bargain — it is a problem waiting to happen. A proper itemised quote should cover groundwork and foundations, the structural elements including any steel beams, brickwork, roofing, windows and doors, plastering, electrics, plumbing, heating, flooring and decoration, along with skip hire, scaffolding and building control fees. When everything is listed, you can compare quotes fairly and you know exactly what you are paying for.
The factors that move the price
Size is the obvious one, but it is far from the only driver. The specification makes an enormous difference — bi-fold doors, roof lanterns, underfloor heating and premium flooring all add up quickly, while a simpler finish keeps costs down. Structural complexity matters too: an open-plan layout that needs large steel beams to remove load-bearing walls costs more than a straightforward box extension. Ground conditions can surprise people, and parts of Horsham sit on clay, which sometimes calls for deeper or reinforced foundations.
Fitted elements are a big variable that many homeowners underestimate. If your extension houses a new kitchen or bathroom, the cost of the units, appliances and sanitaryware sits on top of the building work and can range from modest to substantial depending on your choices. Site access plays a part as well — a terraced house near the town centre where materials have to be carried through the house will cost more in labour than a detached property with side access. And the level of finish you want, from standard to high-end, can shift the total significantly.
The costs people forget to budget for
Beyond the building work itself, there are several costs that catch homeowners off guard. Professional fees for an architect or designer, and for structural engineer calculations, are usually a few thousand pounds and are essential for anything involving structural change. Building control fees and, where relevant, planning application fees need to be accounted for. If your work affects a shared boundary you may need a party wall surveyor. And it is wise to hold a contingency of around ten to fifteen per cent of the build cost for the unexpected, which is especially sensible when opening up older properties where you cannot see everything until work begins.
VAT is another one to factor in. Most domestic extension work is subject to VAT at the standard rate, so make sure you are clear whether the figures you are quoted include it or not. A reputable builder will always be transparent about this.
Single storey or double storey — which offers better value?
If you are weighing up whether to go out with a single storey extension or up and out with a double storey, value per square metre usually favours going bigger. Because the most expensive elements — foundations, roof, scaffolding and groundwork — are largely the same whether you build one floor or two, adding that second storey often delivers a lot more usable space for a relatively modest increase in overall cost. If you have a growing family and need both extra living space downstairs and additional bedrooms upstairs, a double storey extension frequently makes the most financial sense in the long run. It is also one of the more reliable ways to add value to a home in a desirable area like Horsham.
How to budget with confidence
The single best thing you can do is get a detailed, itemised quote based on a proper site visit, rather than relying on a rough phone estimate or an online calculator. A good builder will walk your property, understand what you want, flag anything that could affect the cost, and then set it all out clearly so you can see where every pound goes. From there you can make informed decisions — trimming the specification here, splashing out there — with a full picture in front of you.
That is exactly how we work. We offer free, no-obligation quotations for extensions across Horsham, Broadbridge Heath, Southwater, Roffey and the surrounding villages, with transparent pricing and no hidden extras. What we quote is what you pay, provided the scope stays the same, and if something unexpected surfaces we explain it and agree the cost with you before any additional work happens.
If you are planning an extension and want a clear, honest idea of what it will cost for your home, get in touch today to arrange a free site visit and a detailed quote.