Should I Knock Down and Rebuild? (UK 2026 Decision Tree)

The four comparators

Before reaching for the calculator, the homeowner has four options. Extend the existing house, renovate it back to current spec, knockdown rebuild, or sell and buy something else in the same area. The right answer turns on plot value, end-value ceiling, planning constraints, and how much new floor area is genuinely needed.

1. Extension

Cheaper in absolute terms below ~60 sqm of new space. Standard-rated for VAT at 20%[HMRC 708], which adds £400 to £600 per sqm to the headline before retrofit insulation and party-wall costs land. Best when you need one storey of additional volume and the existing fabric is sound.

2. Full renovation

Strip-back-to-brick renovation often arrives within 15% of new-build cost per sqm once rewire, replumb, retrofit insulation, and structural openings are tallied. 5% reduced VAT applies only to qualifying empty homes[HMRC 708]. Best when the building is listed, in a conservation area, or has structural character worth preserving.

3. Knockdown rebuild

Wins on £/sqm above 90 sqm of new floor area, and decisively on lifetime energy cost because the envelope is built to Future Homes Standard from day one[FHS 2025]. Zero VAT.

0% VAT. New dwelling (VAT Notice 708)

4. Sell and buy up

Adds a SDLT round-trip plus agent fees plus removals; on a £1.2M house that is around £85k to £110k of friction[HMRC SDLT]. Best when the area no longer fits your life, or planning is hostile to replacement dwellings on your plot.

The sequence

Decide in this order. Do not pay anyone until step 1 and 2 are answered.

Plot value vs build-out ratio

On a plot worth £1.2M with a £550k all-in KDR, end-value needs to comfortably exceed £1.75M for the math to work. Use Rightmove sold-prices for comparable post-KDR streets in your postcode.

Planning feasibility (pre-app)

Book a pre-application consultation with your local authority before you pay an architect. Replacement-dwelling policy under NPPF Para 84 [NPPF] is broadly permissive but Green Belt, AONB, and conservation areas tighten the test materially.

Cost the all-in number

Use the on-site calculator. Take the worked example to your architect. Do not commit to Stage 2 fees on a developer-quote-only figure.

Finance + insurance

Self-build mortgage or bridging facility plus site-insurance and structural warranty are non-trivial. Do this before demolition, not after.

Irreversible risks

The four things that cannot be undone:

Planning refusal after demolition. Never demolish before full planning permission is in hand, and read the decision notice conditions carefully.
Asbestos discovered mid-demolition (HSG264 survey is the cheap insurance against this).
Party-wall dispute that delays foundations.
Self-build mortgage pulled at valuation; bridging-loan refinance gap.

Should I knock down and rebuild?