Self Build Mortgage Calculator

Unlike a standard mortgage, the money isn't released as one lump sum. Instead, funds are released in stages as your build progresses, commonly for things like buying the land, completing the foundations, reaching wind and watertight, and finishing the interior. During the build, you typically pay interest only on the amount actually drawn down so far, not the full facility. Once the build is finished, the mortgage usually converts to a standard capital repayment mortgage over a remaining term, similar to any other residential mortgage.

With arrears stage payments, funds are released after each stage is completed and inspected, meaning you need to fund the work yourself first and get reimbursed; this is the more common, and often cheaper, option. With advance stage payments, funds are released before a stage begins, so you have cash upfront for materials and labour, but fewer lenders offer this and rates tend to be higher, since the lender has less security for money that hasn't yet been turned into a completed stage of building work.

Because there's no standard, lender-wide stage schedule. The number of stages, their timing, and how much is released at each one vary hugely between lenders and between projects, depending on the build method, size, and location. Rather than presenting a fabricated "typical" schedule as if it were official, this calculator lets you enter your own plan, or a realistic estimate of one, so the numbers reflect your actual project.

It varies significantly by lender and by whether you choose arrears or advance stage payments; some products ask for as little as 5% towards the land and build costs, while others expect 15% to 25% or more, sometimes calculated against the land purchase specifically rather than the whole project. Check the specific requirement with your lender rather than assuming a fixed percentage.

Many self-build mortgage lenders expect the build to complete within a set window, commonly around 24 months from when the first funds are released, and may charge extra fees, a higher rate, or require a formal extension if it overruns. Delays are common in self-build projects, so it's worth building some contingency into your own timeline and checking your specific lender's policy on extensions before you start.

Many lenders that offer self-build mortgages also offer similar staged-drawdown products for major renovations, conversions and extensions, sometimes marketed separately as renovation or conversion mortgages. The stage-payment and interest-only-during-work mechanics this calculator models are broadly the same, though eligibility and lender criteria differ from a full new-build self-build project.

Yes, typically. A standard buildings insurance policy usually doesn't cover a property under construction, so most self-build mortgage lenders require a specific self-build insurance policy, covering things like site risks, unoccupied property risk, and the works themselves, for the duration of the build. This is a separate cost from the mortgage figures shown in this calculator.