Budgeting for a house extension requires knowing what you can afford before breaking ground. Build costs range from £124,000 up to £750,000+, so having a rough outline helps you decide if your goals match the cash available. An extension updates space for growing families without the hassle or cost of moving. It can add value, potentially netting a profit if you move later, provided you devise a well-thought-out budget and stick to it.
What Drives the Price Tag
Several factors dictate the final bill. Size and openness influence the need for supporting steel. Structural complexity and foundations add up quickly. The biggest variable is specification: bespoke kitchens or top-of-the-range glazed doors cost more but offer a wow factor. Location matters, too, as labour rates differ across the UK.
Construction cost inflation averages around 3%–4% a year. Unless stated otherwise, the figures below cover build and fit-out, not the land or original property. The costs reported here reflect when each project was completed and have not been adjusted for current prices.
Under £150,000
A stone barn in Wiltshire was upgraded with a SIPs extension for £124,000. Rhodri and Meinir Jones transformed their one-bedroom barn conversion into a light-filled family home. The property sits within a listed building’s curtilage, presenting a planning challenge. The design combines flat-roofed and pitched sections to give the impression of several smaller structures. Slate references nearby service buildings, while the contemporary addition contrasts with the original clay-tiled barn. The extension used a structural insulated panels kit costing £13,000. Rhodri and Meinir designed and built much of the project themselves, saving costs by bringing in subcontractors only for specialist work. The project took 35 months to complete at £653 per square metre.
£150,000 to £250,000
A environment architect bought a run-down Victorian cottage in Walthamstow for £162,000. Ian Turkington aimed to create a light-filled, efficient home connected to his garden. The first planning proposal featured a flat-roofed extension, but the local authority deemed it too high. The team developed a second design with a dual-pitch roof and exposed cranked flitch beams. The flitch beam provided steel strength at a lower cost. Ian discovered he needed to replace rotten joists and install a new staircase for £7,000. The finished scheme uses recycled materials, lime render, and natural hemp insulation. The work took 11 months and cost £1,742 per square metre.
Blu Reynolds and Corey bought a ground-floor Victorian flat in Brixton for £171,500. The couple lived in the property for a year before committing to a design. Rising construction costs meant abandoning an initial plan to excavate the cellar for an ensuite and dressing room. They focused the budget on improving the main living areas instead. A new 3.6m-deep rear extension provides a kitchen-living-dining space with London brick walls, bifold doors, and a rooflight. The internal layout created a third bedroom/study and a glazed courtyard. Planning restrictions on roof height meant the living area sits slightly lower than the kitchen. The project took 56 weeks and cost £1,681 per square metre.
Claire and Chris Lunt saw potential in their tired 1950s detached house in Hertfordshire for £180,000. Their ambition for a sustainable renovation had to balance against a limited budget. They initially considered a two-storey extension but dropped the guest bedroom to focus on a single-storey addition. The timber-clad extension runs across the rear and wraps around the side, incorporating part of the existing garage. Built from sustainably sourced timber and clad in blackened larch, it features a sedum roof and large sliding doors. Covid caused material prices to rise, and contractors became harder to find. They adjusted specifications and undertook some building work themselves, including internal strip-out and decorating. The project took five months and cost £1,200 per square metre.
£250,000 to £500,000
Frank and Paloma Gilks discovered a dilapidated Victorian terrace house in West London with structural problems. They aimed to create a spacious family home while retaining original character. Frank designed the scheme himself, taking inspiration from a neighbouring property. The scheme includes a striking side-return extension and a loft extension. The council requested a sloping mansard roof to protect views from a neighbouring cemetery. The couple built the loft extension as a shell due to pay cuts during the start of the Covid-19 pandemic. They salvaged floor joists and reused bricks to keep costs down. The finished 201m² home cost £212,446 (£1,650 per m²) and was completed in seven months.
Nigel and Sue Evans wanted to create a generous long-term home in Edinburgh rather than downsizing. They preserved original features like marble fireplaces and shutters. The rear presented a challenge, with disconnected rooms and changes in floor level. Working with architect Niall Hedderman, they developed a scheme that preserved the layout while introducing a contemporary extension. The original conservatory was removed, and a steel frame was inserted within the existing shell. The design divides the addition into three connected zones, each with its own relationship to the garden. The renovation and extension cost £255,990, with the extension making up £3,600 per m². Despite a four-month delay ordering glazing, the project came in on budget.
Building an extension is a significant investment that reshapes how a home functions and flows. Unlike moving house, which disrupts life entirely, extending allows you to stay in a known environment while adapting the space to specific needs. This continuity often becomes a deciding factor for families, as it avoids the logistical chaos of school transfers and settling into a new neighbourhood, allowing the focus to remain on the physical transformation of the home itself.
Keeping costs under control often requires rethinking initial assumptions. Krystelle and Ed Floyd-Wass bought a run-down 1930s detached house in South East London for £260,000. They preserved original character while creating a modern family home. Architect Helen Yeadon developed a single-storey rear addition with a pitched roof and vaulted interior. The enlarged kitchen-diner features rooflights and heritage-style aluminium-framed glazing. The couple reused materials wherever possible, including original oak flooring and 1930s Bakelite handles. However, unforeseen expenses included a new water main and drainage system. The 180m² renovation and extension cost £260,000 (£1,444 per m²).
£500,000 to £750,000
Clare and Damian Shaw took a chance on a tired 1970s bungalow in Exmouth during lockdown. The three-bedroom property had never been substantially updated, but they saw the opportunity to create a modern family home. The biggest design constraint was the existing ridge height, which could not be raised. Rather than adding another storey, architect Caroline Shortt developed a clever layout, positioning the staircase further back.
The rear extension houses the open-plan kitchen, dining and living area with vaulted ceilings, exposed trusses, and large picture windows. The couple appointed Caroline as project manager as well as architect. Careful costings helped handle post-pandemic price rises, though some cost creep occurred as they upgraded kitchen and bathroom fittings. The 240m² project cost £608,500 (£2,535 per m²) and took 45 weeks to complete.
