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Self-build mortgage calculator
Home Self Build Self-Build Mortgage Calculator

Self-Build Mortgage Calculator

Estimate your borrowing, LTV, stage release schedule and monthly payments across the full build.

How self-build mortgages work

A self-build mortgage is fundamentally different to a standard residential mortgage. Because your home does not exist as security at the point of application, lenders release funds in stages as the build progresses rather than as a single lump sum. During the build you pay interest only on the amount drawn down so far - so costs start low and rise as each milestone is signed off. Once the build is complete and a completion certificate is issued, the mortgage typically converts to a standard capital repayment product, at which point many self-builders remortgage onto a more competitive rate.

Key facts
  • › Funds released in stages, not upfront
  • › Interest only during the build phase
  • › Converts to repayment on completion
  • › Max 85% LTV based on end value
Arrears vs Advance release
  • › Arrears: you fund each stage, claim back after sign-off
  • › Advance: lender pays before each stage begins
  • › Arrears = lower interest cost, needs cash reserves
  • › Advance = easier cashflow, higher overall interest
What to budget beyond this calc
  • › Professional fees: typically 10-15% of build cost
  • › Self-build insurance and structural warranty
  • › Contingency fund: minimum 10-15% of build cost
  • › VAT reclaim after completion (HMRC DIY scheme)

Your Project Costs

£
£
£
£

Mortgage Terms

years
%
%
Arrears: you fund each stage, then claim back after sign-off.

Stage Release Schedule - adjust % to match your lender. Must total 100%.

Stage % of build cost Release amount
Land Purchase
Site acquisition, SDLT & legal
 Land cost -
Foundations Complete
Groundworks, drainage, slab
 % -
Wall Plate / Superstructure
Walls built to wall plate level
 % -
Wind & Watertight
Roof on, windows & doors in
 % -
First Fix
Plastering, first-fix electrics & plumbing
 % -
Practical Completion
Second fix, snagging, sign-off
 % -
Important: Indicative estimates only - not financial advice. Always consult a qualified self-build mortgage adviser before making any decisions.

Self-Build Mortgage FAQs

What is the maximum LTV on a self-build mortgage?
Most self-build lenders cap borrowing at 85% of the estimated completed value of the property - not the build cost. Some lenders offer up to 95% LTV on the land purchase, then reduce to 85% for the build stages. The higher the LTV, the fewer lenders will be available to you and the higher the interest rate is likely to be. A deposit of at least 20-25% of total project cost gives you access to the most competitive products.
What is the difference between arrears and advance stage release?
With arrears release, you fund each build stage from your own cash reserves, then claim back from the lender once a building inspector has certified the stage is complete. This means lower interest costs as drawdown is slower - but you need sufficient liquidity to fund stages upfront. With advance release, the lender pays funds before each stage begins, so you do not need the same cash reserves. Interest accrues from the moment each tranche is released, so the total interest cost during the build is higher. Advance products tend to have higher rates and fewer lenders offer them.
Do I need a self-build mortgage or can I use a standard mortgage?
A standard residential mortgage cannot be used to finance a self-build project because the property does not exist as security at the point of application. You need a specialist self-build mortgage that releases funds in stages. Once your home is complete and has a completion certificate, you can remortgage onto a standard residential product - often at a significantly lower rate - which is why many self-builders plan a remortgage from the outset as part of their exit strategy.
Can I get a self-build mortgage if I already own a home?
Yes, but lenders will assess your total debt exposure across both properties. If you have an existing mortgage, the repayments will be factored into affordability. Some people release equity from their existing home to fund the deposit and early stages of the self-build, then sell once the new property is complete. If you sell your existing home before moving in, you will need to arrange temporary accommodation for the build period - a cost worth factoring into your budget. SDLT second-home surcharges may also apply on the land purchase if you own another property at the time.
How are self-build mortgage payments calculated during the build?
During the build phase, almost all self-build mortgages are interest-only. You pay interest only on the amount that has been drawn down so far - not on the full approved mortgage amount. As each stage is signed off and a new tranche is released, the drawn-down balance increases and so does your monthly interest payment. Once the build is complete and the mortgage converts to standard repayment, you begin paying both capital and interest on the full outstanding balance. This calculator shows the average monthly interest during the build and the full monthly repayment after completion.
What other costs should I allow for that are not in this calculator?
This calculator covers your core land and build financing only. Budget separately for: professional fees (architect, structural engineer, quantity surveyor, planning consultant - typically 10-15% of build cost); self-build insurance (site insurance, public liability, structural warranty); planning application fees; building control fees; SDLT on the land; broker and lender arrangement fees; temporary accommodation if you are selling your existing home; landscaping and external works; and a contingency fund of at least 10-15% of build cost for cost overruns, which are common on self-build projects.