Building an extension on a house is a good way to gain space without having to move, but it’s a big home renovation that doesn't come cheap. So how much does an extension cost?
The cost of an extension is determined by the size, structural requirements and location (more details below). Also, design and materials: square shapes and concrete are relatively straightforward, but costs stack up for complex shapes or timber frames. What you do with it also counts: kitchens and bathrooms cost more than other parts of the house.
You also need to factor in fees for planning permission and Party Wall Agreements, if applicable. Ditto building regulations and professional services like architects, surveyors and builders.
When digging foundations, the quality of the ground impacts on cost as poor soil or a sloping site require more complex engineering.
If excavations or construction are near walls or boundaries shared with neighbours, you may need to pay £700-£1,500 for a surveyor to draw up a Party Wall Agreement. If there’s no friction, however, you can save by serving notices yourself.
After that, pricey parts of the build include installing steel beams, drainage, gas and water pipes and the roof. Gables tend to be cheapest, followed by flat roofs and slopped ‘hips.’
The quality of materials is also important. Kitchens should be fitted to a high standard and last years. For a living room, however, your biggest expense will be the furniture.
As a rough estimate, a single-storey rear extension costs anywhere from £60,000 to £130,000. Stick to the limits stipulated for Permitted Development Rights (PDRs) to save planning permission fees.
If you’re watching the pennies, orangeries often work out cheaper than a traditional extension. Like a conservatory, they’re light and airy, but their partial brick construction makes them more suitable for year-round living.
While a two-storey extension obviously costs more, it shouldn’t be double as the foundations and roof are the same; you’re just adding a ceiling, floor and fittings. It depends on the use, of course: an extension with an upstairs bathroom and ground floor kitchen will set you back £90,000 - £140,000 compared to £50,000 - £80,000 for other rooms.
Note that you can’t extend two-storeys as far under PDRs. Check out our article for more information about what size extensions you can build without planning permissions.
‘Side returns’ or ‘side infills’ are a great way to repurpose dead space down the side of an L-shaped house. You’d be amazed how even the narrowest alley transforms the interior once knocked through.
Side returns tend to come under PDRs and are usually smaller than rear extensions, so they cost less. And you don’t have to give up valuable garden space.
Kitchens in older houses are often cramped, a hangover from the days when they were purely functional. Now, though, they’re often the heart of the home and a popular choice to extend and remodel.
Layout, materials and appliances all influence cost. Adding a side return dining area is relatively cheap, whereas a rear extension with comprehensive kitchen remodel and new features like an island is much more expensive.
Fixtures and appliances will add £5,000 to £25,000 to your budget, depending on the quality. Bespoke work and smart appliances cost a lot more than off-the-shelf units and white goods from major retailers.
Kitchen extensions usually come under PDRs, but always check first.
Often called a loft conversion, a loft extension is a cost-effective way to optimise existing space by building up instead of out, particularly if you don’t have much land to spare. Also, you don’t have to pay to dig foundations or knock down walls.
As a guide, a loft extension costs £35,000-£50,000. The main determinant of price is whether you need to alter or raise the roof. If you already have head height, you can just add rooflights and convert the space. If not, you’ll need a more costly dormer, hip-to-gable, gable-to-gable or mansard loft conversion, which may require planning permission.
Again, how you use the space is important. Bedrooms with en-suite bathrooms are popular, but you need to factor in building internal walls and installing plumbing and sanitary ware on top of renovation costs.