Chapter 9 of 10 · Self-build, extending and renovating · Self-build mortgage calculator
Self-build mortgage calculator
How does a self-build mortgage pay out?
Checked 21 Oct 2026the usual self-build stage schedule and the Help to Buy rules.
A self-build mortgage is paid out in stages as the house goes up, not all at once. Enter the site, the build cost and the loan. The tool dates each stage, works out the interest you pay during the build, and shows the cash you need before the lender pays anything.
The site, the build and the loan
What will the build cost, and what are you borrowing?
The stages
How the lender pays the loan out
A self-build mortgage is paid in stages as the build goes up. Each stage is a share of the loan, paid when an engineer or architect signs that stage off. Change the shares and the months to match your lender's schedule.
Stage 1: Foundations and floor
Stage 2: Walls to wall plate
Stage 3: Roof on and weathertight
Stage 4: First fix (plumbing, wiring, plaster)
Stage 5: Completion
The shares add up to 100%.
The stage shares are a template. Your lender's schedule, your engineer's certificates and your builder's invoices set the real dates and amounts. The tool does not say what a lender will offer or what the finished home is worth.