Category
Should I extend or move? The numbers for London homeowners
Chris Cheuk
6 min read
Summary
Moving typically costs 8 to 10 per cent of a property's value in transaction costs alone, before you gain any additional space. Extending converts that same money into square metres, provided your street has room above its ceiling price.
Key takeaways
Moving costs 8 to 10 per cent of property value in stamp duty, agent fees, legal costs and removals, and buys no additional space.
Extending converts transaction cost into floor area, but only recovers at resale if your street has headroom above its ceiling price.
The ceiling price test is the decisive one: if the best house on your street has sold within about £25,000 of your value plus your spend, the money will not come back.
Resale return is the wrong measure for most people, because it ignores the years of use before any sale.
Extending rarely makes sense if you intend to move within three years, or if it would leave under 25 square metres of usable garden.
The short answer
For most London homeowners in 2026, extending is the better financial decision, because moving spends a very large sum without producing a single additional square metre.
Transaction costs on a London move, stamp duty, estate agent fees, legal work, removals and mortgage arrangement, routinely reach 8 to 10 per cent of the property value. On a £900,000 house that is £72,000 to £90,000 spent to arrive somewhere else. The same money spent on a well-planned extension buys around 20 to 25 square metres of new space on the house you already own.
That is the case in outline. The exceptions are real, and they are worth taking seriously before you commit.
The ceiling price test
Every street has a ceiling: the price above which buyers stop paying, no matter how good the house is. It is set by the location rather than by the property.
The test is straightforward. Take the best house on your street, the largest and most improved, and find what it sold for. Compare that against your current valuation plus what you intend to spend. If your total lands within roughly £25,000 of that ceiling, or above it, the extension will not recover at resale. You will have built something excellent that the street will not pay for.
This is the single most useful piece of analysis before you spend anything, and it takes an afternoon on the Land Registry and the portals.
Resale return is the wrong headline number
Most cost guides lead with return on investment at resale, which is a misleading way to frame the decision for someone who intends to live in the house.
A resale figure ignores the ten or twenty years of additional space you use before any sale happens. It ignores the stamp duty you did not pay by staying. For a family that needs a bigger kitchen now, the relevant question is not what percentage comes back in 2040, it is whether the space solves the problem and whether the money is available.
Treat resale recovery as a floor rather than a target: enough that you are not destroying value, not so much that it becomes the reason for building.
Where extending does not make sense
Three situations where the answer is genuinely no.
If you intend to move within three years regardless, the disruption and the unrecovered cost rarely justify it. An extension takes six to twelve months from first conversation to completion, and you would be living through the worst of it for a fraction of the benefit.
If the extension would leave you with under about 25 square metres of usable garden in a family-house market, the loss of outside space can cost you more at sale than the internal area gains. London buyers with children price gardens seriously.
If your street is at its ceiling, as above.
The costs people forget on both sides
On the moving side: stamp duty is the obvious one, but agent fees at 1 to 2 per cent plus VAT, legal costs on both transactions, survey fees, removals and the mortgage arrangement all add up quickly. So does the risk of a chain collapsing after you have spent on searches and surveys.
On the extending side: professional fees at around 15 per cent of construction cost, statutory fees and surveys at around 3 per cent, and a contingency of at least 10 per cent. Current thermal and airtightness standards add cost to any new envelope, and on a terrace or semi you will have party wall surveyor fees for each affected neighbour.
Neither list is a reason not to act. Both are reasons to compare like with like rather than comparing a builder's headline rate against an estate agent's valuation.
How to decide in a week
Establish the ceiling price on your street. Get an indicative build cost for the space you actually need rather than the space you might want. Add fees, statutory costs, contingency and VAT to reach a real total. Then price the move properly, including every transaction cost, and compare the two figures against what each one gives you.
If the numbers are close, the deciding factor is usually not financial. It is whether you want to stay where you are.
Frequently asked questions
Is it cheaper to extend or move in London?
Usually cheaper to extend. Moving costs 8 to 10 per cent of property value in stamp duty, agent fees, legal work and removals, and delivers no additional space. The same sum spent on an extension typically buys 20 to 25 square metres on the house you already own.
What is a ceiling price and why does it matter?
A ceiling price is the level above which buyers on your street stop paying, regardless of how good the house is. If your current valuation plus your intended spend lands near or above it, the extension will not recover at resale, however well it is built.
How much value does an extension add to a London house?
It depends far more on your street's ceiling price than on the extension itself. A well-planned extension below the ceiling generally recovers a substantial share of its cost. Above the ceiling it recovers very little, because the location rather than the property sets the limit.
When does extending not make sense?
When you intend to move within three years, when your street is already at its ceiling price, or when the extension would leave under about 25 square metres of usable garden in a family-house market. In that last case the lost outside space can cost more than the internal gain.
How long does an extension take from start to finish?
Six to twelve months from first conversation to completion is typical, covering design, consent, tender and construction. Construction alone runs roughly twelve to sixteen weeks for single-storey work and longer for two-storey. Consent route and party wall matters drive most of the variation.
Should I decide based on resale return?
Only partly. A resale figure ignores the years of additional space you use before any sale, and the stamp duty you avoid by staying. Treat recovery as a floor, enough that you are not destroying value, rather than as the reason for building.
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