Use this self build mortgage calculator to plan the finance for building your own home. Enter the land cost, build cost and the share the lender will fund, and it shows the maximum loan, the money you need to put in, the interest during the build and the monthly payment once the house is finished.
Your self build mortgage
- Maximum loan
- –
- Your own money needed
- –
- Interest during the build
- –
- Monthly payment after the build
- –
In the example, land at £120,000 and a £250,000 build, with 75% funded, gives a loan of £277,500. You need £92,500 of your own money. Interest during a 12-month build is about £10,536, and afterwards the payment is £1,704.09 a month at 5.5% over 25 years.
How to use the self build mortgage calculator
- Enter the land cost, or its value if you already own it.
- Enter the build cost, including fees and VAT you cannot reclaim.
- Enter the lender’s maximum share, often 75% of costs.
- Enter the rate, build time and term.
How self build mortgages work
Self build mortgages release money in stages as the build progresses, usually after the land purchase, foundations, wall plate, watertight and completion. With an arrears mortgage, money comes after each stage is finished and valued. With an advance mortgage, money comes at the start of each stage, which helps your cash flow.
During the build, you usually pay interest only on the money released so far. When the home is finished, the loan switches to a normal repayment mortgage or you remortgage.
Costs to plan for
- Land, legal fees and stamp duty on the plot.
- Planning permission, architect, structural engineer and building control.
- Warranties such as a structural warranty, which most lenders require.
- Site insurance and utility connections.
- A contingency of 10% to 15%.
New homes built by self-builders can reclaim VAT on many materials (GOV.UK: VAT on building a new home). Council planning departments must also keep a Right to Build register of people looking for plots.
Self build mortgage calculator tips
Lenders may lend on the land value plus build costs or on the finished value, whichever is lower. Use the self build mortgage calculator with a cautious build cost and allow for delays, because extra months of interest add up.
Related calculators
Check your deposit plan with the mortgage deposit calculator, or the stamp duty on a plot in Wales with the stamp duty calculator wales.
Self build mortgage calculator: worked example
Sam buys a plot for £120,000 and plans a £250,000 timber frame home. The lender funds 75% of costs, so Sam needs £92,500 in cash. During a 12-month build, the interest is about £10,536. Once finished, the home is valued at £480,000, so Sam can remortgage to a standard deal at a much lower loan to value.
Frequently asked questions
How much deposit do I need for a self build mortgage?
Usually 25% of land and build costs, though some lenders go up to 85% to 90% with an advance product.
Do I pay stamp duty on a self build?
Only on the land purchase, at the residential or non-residential rate depending on the plot.
Can I live elsewhere during the build?
Yes, but budget for rent or your current mortgage at the same time as build interest.
Is the self build mortgage calculator a quote?
No. It is a guide to plan costs; lenders set their own limits and stage releases.
About this calculator
This self build mortgage calculator assumes the build money is released evenly each month. Guidance was checked against GOV.UK and lender information on 9 November 2026. Results are estimates.