Good Houses
Extend or move calculator
Step 1 of 4
Where's your current home?
We need this for local build costs, and because stamp duty rules differ across England & Northern Ireland, Wales and Scotland.
This is the one nearly everybody weighs up at some point, usually when a new baby arrives, a parent comes to live with you, or the kitchen finally stops working for the family.
The figures above compare the two routes on cost per square metre, which is the part that can be calculated. What no calculator can price is the rest of it. An extension runs for months, and you will not have full use of your home while it does. Moving has its own version: the surveys, the solicitors, and the chain, which is where a great many sales quietly fall apart.
If you like where you live and have garden to give up, extending often wins. If your house simply cannot become what you have in mind, no amount of spending will change that.
How the two options are compared
This one runs two calculations and puts them side by side, because the question is not what each option costs but which gets you more space per pound.
The extend side uses the same method as our extension calculator: a cost per square metre for the type of extension, adjusted for finish and local labour, plus the extras and statutory costs that apply. Against that it estimates what the work would add to your property at your area's prices, and applies a ceiling check: if the extension would push your home past what buyers in your area will pay, it says so, because the last part of that spend is unlikely to return.
The move side adds up what moving actually costs: stamp duty at the correct regime for your destination, estate agent fees, conveyancing for both the sale and the purchase, a survey, removals and mortgage fees. Those are real costs that disappear the moment the transaction completes.
Both are then reduced to the same measure, cost per square metre of extra space, so they can be compared honestly. Extending is often cheaper per square metre; moving buys space you do not have to live through building.
The result is always a range, never a single figure. A precise number would be a false promise, because two builders quoting the same job honestly can land a long way apart. Everything is rounded rather than shown to the pound, for the same reason.
Every figure behind this comes from a table we maintain by hand and review on a schedule. The date of the last review is shown on this page, so you can judge how current it is rather than take our word for it.
What both sides of the comparison cover
On the extend side:
- The full build cost, at the type and finish you choose
- Statutory costs and a contingency
- Estimated value added at your area's prices, with a ceiling check
On the move side:
- Stamp duty, at the right regime for the destination
- Estate agent fees
- Conveyancing for both the sale and the purchase
- Survey, removals and mortgage fees
What neither side of the figure includes
Not in either side:
- The cost of living somewhere else while an extension is built
- Schools, commute and neighbours, usually the reason people move, and not reducible to pounds per square metre
- What happens to your mortgage rate if you move now rather than later
- Capital gains, where the property is not your main home
- The time and disruption of building, which is real even though it is not a cost
Why location pushes the two options apart
Both sides move with location, in opposite directions. Building costs follow local labour rates. Moving costs follow local property prices. A 10% larger house costs far more to step up to in an expensive area than a cheap one, and stamp duty is banded, so the gap widens as prices rise.
This is why the answer genuinely differs by area rather than being a national rule of thumb. In some places extending is obviously cheaper; in others the ceiling price makes it a poor investment however cheap the build.
Reading the comparison
Treat the cost per square metre as the comparison and the rest as context. If extending is cheaper per square metre but the ceiling warning is showing, that means you can have the space but should not expect to get the money back.
And be clear with yourself about what is really driving the decision. If you are moving for a school catchment or a shorter commute, no cost-per-square-metre figure should overrule that. This calculator is for the cases where the two options are genuinely close and money is the deciding factor.
Common questions
Which is usually cheaper?
Extending, per square metre, in most areas. Moving costs are largely transaction costs that buy you no space at all. But "usually" is doing a lot of work in that sentence, and the ceiling check is what turns it around: if your house is already near the top of what your street achieves, extending stops paying back.
What is the ceiling price?
The point above which buyers in your area stop paying, regardless of how good the house is. Every street has one. Spending past it is a lifestyle decision rather than an investment, and the calculator flags it rather than letting the numbers imply otherwise.
Does it account for the disruption of building?
No, and it cannot. Months of building work with a family in the house is a genuine cost that does not convert to pounds. It is worth weighing separately, and it is the most common reason people who could extend decide to move.
Is this a quote?
No. It is an estimate built from typical costs, and no contractor is bound by it. Use it to know whether a quote you have been given is roughly where it should be, and to walk into that conversation knowing what questions to ask.
Why do you show a range instead of one number?
Because one number would be misleading. The answer to "what will this cost" is a range, and where you land inside it depends on your property, your area and who you hire. Any tool giving you a single figure to the pound is telling you something it cannot know.
Do you save what I type in?
No. The calculation runs and the answers are discarded. Nothing is written down unless you explicitly ask us to email or save a result. We do not build a profile from calculator use.
How current are the figures?
The date of the last review is shown on this page. The costs behind the calculator are maintained by hand rather than scraped, and reviewed on a schedule.