This is a collaborative post
Every extension conversation starts the same way. Someone says "we've outgrown this house", someone else says "moving costs a fortune", and within ten minutes you're sketching a kitchen-diner on the back of an envelope.
What's usually missing is the maths. Extending can be an excellent decision. It can also be the most expensive way to end up in a house you still don't like. The difference comes down to four numbers: what your street will pay, what the build really costs, what your site makes it cost, and what moving would cost instead.
Start With the Ceiling Price on Your Street
Every road has a number it won't go past. You can feel it when you scroll through sold prices: the four-beds and the three-beds converge, and nothing has sold above a certain figure in five years, however nicely it was finished. That's your ceiling value. Spend past it and the money doesn't come back.
Space does add value, but less dramatically than people assume. Nationwide's research puts it plainly: a 10% increase in floor space adds around 5% to the value of a typical house. The bigger gains come from what the space is, not how much of it there is. The same analysis suggests a loft conversion or extension delivering a proper double bedroom plus a bathroom can add as much as 24% to a three-bed, one-bathroom house. An extra bedroom and bathroom, then, moves you into a different search bracket. A larger kitchen usually doesn't.
Do this before you speak to anyone: find three houses on or near your street that already have the extension you're planning, and see what they actually sold for. That gap between their price and your current value is your entire budget for the project, including fees, and including the parts nobody quotes for.
What the Build Reliably Costs
The Federation of Master Builders puts the average single-storey extension in the UK at between £1,800 and £3,000 per square metre. London, awkward access and a high specification push past that comfortably. So a modest 20 square metre rear extension is a £36,000 to £60,000 build before anything else. Loft conversions land in a similar range; trade cost guides for 2026 put a typical rear dormer somewhere around £45,000 to £75,000, more in London once a bathroom and structural work are included.
Then come the costs that never make the envelope sketch:
- Design fees: An architect or designer to draw the scheme, and a structural engineer for the calculations.
- Planning fees, where they apply: A householder application in England costs £548 from 1 April 2026. Prior approval for a larger home extension is £249.
- Building control: Either through the council or a registered building control approver, the role that replaced the old 'approved inspector' in April 2024.
- Party wall surveyors: Only if a neighbour dissents, but budget for it in case they do.
- Finishing the space: Kitchen, bathroom, flooring, decorating, and putting the churned-up garden back together.
- VAT: Charged at the standard rate on most domestic building work.
Add 10 to 15% contingency on top and treat it as spent. In my experience, the projects that stay calm are the ones where the contingency was budgeted from day one rather than found later.
 |
| Photo credit Brett Jordan via Unsplash |
Your Site Will Decide More Than Your Drawings Do
Two identical semis, same street, same brief. One extension comes in on budget. The other adds £18,000. The difference is almost always underground or invisible: a garden that falls half a metre from house to fence, a shared drainage run crossing exactly where the foundations want to go, a boundary that isn't where the fence is, a rear wall that bows out 90mm along its length, or a neighbour's foundation you now have to underpin against. None of that shows up on a Land Registry title plan, or on measurements taken with a laser distance meter on a Saturday morning. This is where projects quietly go wrong.
A designer works from approximate dimensions and assumes square corners. The drawings get priced, permission is granted, and then the builder sets out on site and finds the reality is 100mm off. Steel gets re-ordered. Rooflights don't fit the opening. The design goes back for redrawing, and because the approved drawings no longer match what's being built, the paperwork has to catch up too.
Why Architects Ask for Accurate Existing-Condition Drawings
A good designer will ask for a survey of what's already there before drawing anything. Not to pad the fees, but because they are about to design a structure that has to meet your existing building exactly, in three dimensions, with tolerances of millimetres. That survey is a measured building survey: plans, elevations and sections of the house as it genuinely is, with levels, drainage and boundary detail tied in.
Terrain Surveys, working out of Hertfordshire since 2004, is one firm producing this sort of measured building survey across London and the wider UK, and it also does laser scanning if your architect prefers a point cloud to model from.
A few hundred pounds spent there is the cheapest insurance available against a redesign halfway through a build. If your plot slopes or your boundary is contested, a topographical survey of the garden is worth adding at the same time. Cheaper as one visit than two.
Planning and Party Walls: The Parts That Eat Your Timeline
Plenty of extensions don't need planning permission at all. Under permitted development rights in England, a single-storey rear extension can typically project 3 metres from the original rear wall on a terrace or semi, or 4 metres on a detached house. The larger home extension scheme takes that to 6 metres and 8 metres respectively, via a prior approval application where neighbours get consulted. It doesn't apply in conservation areas, National Parks or other designated land, and it doesn't apply if previous owners have already used up the allowance.
Loft conversions have their own limits: 40 cubic metres of additional roof space for a terrace, 50 for a semi or detached house, including any previous roof extensions, with nothing above the existing ridge.
Permitted development is not a free pass. Building Regulations still apply, and so does the Party Wall etc. Act 1996. That last one catches people out. If you're excavating within 3 metres of a neighbour's building and going deeper than their foundations, or within 6 metres and cutting a 45-degree line down from them, you must serve notice at least a month before you dig. Work to a shared wall itself needs two months' notice. A neighbour who dissents isn't blocking you, but you're now both appointing surveyors, and that is weeks and hundreds of pounds you hadn't planned for.
Rules differ in Scotland, Wales and Northern Ireland, so check locally rather than assuming the English position.
Planning and Party Walls: The Parts That Eat Your Timeline
Ten to sixteen weeks with no back wall. A microwave on the landing. Dust in the wardrobe. Skip permits, a driveway full of sand, and a builder who disappears for eight days when another job overruns. Some families take it in their stride. Some spend the money on a rental for three months, which quietly adds several thousand pounds to the project.
Be honest with yourself about which one you are, and about whether the household can absorb a six-week delay in the middle of it. Delay is normal rather than exceptional.
When Moving Wins
Compare properly and moving often looks better than its reputation suggests. The HomeOwners Alliance puts the average cost of moving in 2026 at around £13,018 on an average UK property of roughly £292,000, with estate agent fees averaging 1.42% including VAT. Stamp duty is the variable that hurts: since 1 April 2025 the nil-rate threshold in England and Northern Ireland is back to £125,000, with 2% on the slice to £250,000 and 5% above that to £925,000. Even so, £13,000 to £25,000 to move buys you a house that already has the space, already has the layout, and doesn't need a party wall award.
Moving usually wins when: your project would push you past the street's ceiling value, the extension you need isn't achievable within permitted development or planning policy, the site conditions have already added a five-figure sum to the estimate, or the space you actually want is a different location rather than more square metres.
Sometimes the decision isn't a leisurely one. An inherited house that needs £40,000 of work before it's mortgageable, a chain that collapsed, a separation with a deadline attached, a property no lender will touch. In those situations the open market is slow, and six months of viewings while you carry the costs is its own kind of expensive.
That's the gap direct buyers fill. Property Rescue, which has been buying homes for over 20 years, is one of them, and can sell your house fast with no viewings, no chain and no agent fees, typically in 7 to 28 days.
It's worth knowing how the trade-off works before you go looking. Cash buyers pay below open-market value, so the question is whether the discount is smaller than the cost of the delay, the fees and the risk you'd otherwise carry. For some people, the discount is worth paying. For others it plainly isn't, and a normal sale is the right answer.
The Test That Settles It
In the end it comes down to two totals. The first is your build cost, all in, including contingency and living costs, set against the honest value uplift on your street. The second is your moving cost, all in, set against what the extra space would cost you in a house that already has it.
Write both down. Whichever is smaller is your answer, and it is usually clearer on paper than it ever is in conversation.
Build if the extension makes financial sense and you'd still want it even if it added nothing to the value. If it only stacks up because you're assuming the survey comes back clean and nothing overruns, get the survey first. The numbers rarely lie. It's the envelope sketch that does.
