Enter the contingency you plan to hold for overruns.
Market value of the finished property.
Enter the share of the project you will fund yourself.
Use a dated quote for the build stage.
How the self build mortgage calculator works
The self build mortgage calculator adds the plot cost, the build cost and a contingency to give the total project cost, then takes off the deposit to show the loan needed. It compares the loan with the expected value of the finished home and estimates interest during the build, with no credit search.
The interest estimate reflects that self build mortgages are drawn in stages, so interest builds up as each stage is released rather than on the full loan from the start.
How much can be borrowed on a self build mortgage
How much can be borrowed on a self build mortgage depends on income and outgoings, as with any mortgage, and on the lowest of the project cost, the finished value and the lender's limits on the land and each stage. The calculator shows the loan the project needs, not what a lender will offer.
Lenders also check the plot, the plans and the warranty, so a lender may lend less than the calculator suggests. Testing a larger deposit or contingency shows how the loan-to-value changes. How the stages and criteria work is explained in the guide to how self build mortgages work.
Choosing figures to test
The build cost usually comes from a quantity surveyor's estimate or builder's quote, and the finished value from local sales of similar homes. The build-stage rate should come from a dated quote, because self build pricing varies between lenders and with the drawdown method, arrears or advance.
A contingency for overruns is worth including, because groundworks, weather and material prices can all push costs up. Lenders often ask for proof of contingency funds.
Using the results
The result is an estimate based on the figures entered. A lender's offer depends on its valuation of the plot and the finished home, its checks on income and the project, and its stage release rules. Stage valuation and arrangement fees add to the cost, so they should be included in the budget where known.
Self builders can reclaim VAT on many materials through the HMRC DIY housebuilders scheme, which the calculator does not include. Comparing lenders on the self build mortgage lenderspage is a common next step.
Frequently asked questions
Does the calculator affect my credit score?
No. It does not search a credit file.
Can it tell me how much a lender will lend?
No. It shows the loan the project needs. Lenders decide after checks.
Why is interest lower than on a full loan?
Self build mortgages are drawn in stages, so interest builds up as funds are released.
What build-stage rate should I enter?
A rate from a dated quote for a similar project.
Does it include VAT reclaims?
No. VAT refunds under the DIY housebuilders scheme come after completion.
Does it work for kit homes?
Yes, as an estimate, using the kit and erection costs as the build cost.
Related articles
Practical guides and explainers from our blog — relevant to this mortgage type.
- Drawdown
- Staged
- Typical build time
- Varies by project
- VAT reclaim
- Available (new builds)
- Contingency
- Set by your budget
Remortgage onto a standard product once building is complete.