The Question Behind the Question
Most clients who ask this have a house that no longer works: poor insulation, awkward plan, low ceilings, a roof that needs doing anyway. The instinct is to extend, because it feels smaller and cheaper. Sometimes it is. Often it is not.
The Cost Comparison, Honestly
A large extension plus a full renovation of the existing house typically lands at £2,200 to £3,200 per square metre across the whole floor area, and you still have the original structure, its walls, its floor levels and its foundations.
A new house is £2,600 to £4,000 per square metre plus demolition of £25,000 to £70,000, but eligible works are zero rated for VAT rather than charged at 20 percent. On a project of any scale that VAT difference closes a surprising amount of the gap, and you end up with new everything, a modern thermal envelope and a ten year structural warranty.
When Rebuilding Wins
- The existing house has already been extended once or twice and a third addition would look like one.
- Ceiling heights, floor levels or structural walls make the plan you want impossible.
- The thermal performance is poor enough that you would be replacing windows, insulation, heating and wiring anyway.
- The plot supports a materially better house than the one standing on it.
- You want a low energy home, which is far easier to achieve from scratch than by retrofit.
When Extending Wins
- The house has character worth keeping, particularly period property in a conservation area.
- The existing structure is sound and the plan mostly works already.
- Green Belt or conservation policy would allow an extension but not a materially larger replacement.
- You need to stay living there, which is far more realistic during extension work.
- The budget suits a targeted project rather than a full rebuild.
The Planning Test That Decides It
In the Green Belt, a replacement dwelling must not be materially larger than the house it replaces, measured as each local plan defines it. That single test frequently settles the argument, which is why we measure the existing building and read the policy before offering an opinion. Our Green Belt guide sets out how those tests work.
Outside the Green Belt, the question is usually design and street character rather than volume, which gives far more freedom.
How to Settle It Properly
Price both. We routinely run parallel appraisals: a replacement dwelling against an extension and renovation, with realistic figures for each and a clear view of the planning risk on both. Start with the project calculator, then a feasibility study if you want the comparison in writing.
Our new build homes and double storey extensions pages carry guide costs for each route.
We price both options for clients more often than people expect, and the rebuild wins more often than they expect too.
Key takeaways
- Extension plus renovation runs £2,200 to £3,200 per square metre, a new house £2,600 to £4,000 plus demolition.
- New builds are zero rated for VAT while extensions are charged at 20 percent, which narrows the gap.
- In the Green Belt, the materially larger test on replacement dwellings usually decides the answer.
- Rebuilding suits tired houses on good plots and anyone wanting a genuinely low energy home.
- Price both routes properly before committing, because the intuitive answer is often the wrong one.
About the author
Kevin Edge founded Mosaic Bespoke and has spent more than two decades in planning, design and construction, working with private clients and landowners across Hertfordshire, Buckinghamshire, Bedfordshire and North London.