Self-build reality — 15–20% is standard.
Market value of the finished property.
Self-build lenders often want 25%+.
Typically 1–2% above standard residential.
What is a self-build mortgage?
A self-build mortgage funds a home you are building, rather than one that already exists. The lender releases the money in stages as the build reaches agreed points, instead of paying the full amount on day one. You usually need a deposit of about 25% of the whole project, covering the land, the build and a contingency.
The interest rate during the build is typically 1–2% above a standard residential mortgage, because the property is not finished security. Most borrowers remortgage onto a normal home loan once the building is complete and a lower rate is available. This calculator estimates the stage finance from the project cost, deposit and rate you enter. It does not confirm that a lender will fund the build.
A UK self-build with total costs of £437,500 and an end value of £500,000 creates £62,500 in equity, needing a £328,125 mortgage drawn in stages.
How a self-build mortgage works
Stage releases
Most UK self-build lenders release funds in 5–6 stages: purchase/foundations, wall plate, watertight, plastered, second fix, final. Arrears-basis lenders release after inspection; advance lenders release upfront for each stage.
Contingency is critical
15–20% contingency is standard. Groundworks, weather delays, material price rises and scope creep routinely push UK self-builds over budget. Lenders often require proof of contingency funds before approving.
Frequently asked questions
How does a self-build mortgage work?
The loan is released in stages as the build progresses (foundations, wall plate, watertight, plastering, completion). Most lenders release on an 'arrears' basis — you fund each stage, then claim. A handful offer 'advance' stage releases which fund upfront.
What deposit do I need?
UK self-build lenders typically want 25%+ of total project cost (land + build + contingency). A few accept 20%. Owning the land already with no mortgage effectively contributes to the deposit.
Are self-build rates higher?
Yes, typically 1–2% above standard residential during the build, reflecting higher lender risk. After completion, most borrowers remortgage onto a standard residential product at better rates.
Can I reclaim VAT on a self-build?
Yes — HMRC refunds VAT on most new-build materials and some services. Submit a DIY Housebuilders' VAT reclaim within 3 months of completion. Extensions and conversions have different rules.
How much contingency should I budget?
15–20% of build cost is standard. UK self-builds regularly run 5–15% over budget on materials, labour and unforeseen groundworks. Lenders often require evidence of contingency funds before approving.
Reviewed by the Mortgage118 editorial team. Build-year interest is an estimate assuming 50% of the loan drawn on average over 12 months. Individual stage-release schedules vary. Provides an estimate only.
- Drawdown
- Staged
- Typical build time
- 12–18 months
- VAT reclaim
- Available (new builds)
- Contingency
- 15–20% standard
Remortgage onto a standard product once building is complete.