Self-Build Mortgage Calculator — Staged Drawdown & Total Costs

A self-build mortgage releases funds in stages as your build progresses, rather than as a lump sum. There are two types: arrears (funds released after each stage is complete and inspected) and advance (funds released before each stage begins). Arrears is more common with lenders but requires more personal cash flow during the build.

Most lenders offer up to 75-85% of the land purchase price and build costs combined, or up to 60-75% of the final estimated value — whichever is lower. The final borrowing amount converts to a standard mortgage on completion.

When you self-build, you only pay Stamp Duty Land Tax (SDLT) on the land purchase price, not the completed property value. A house worth £500,000 when complete might only have land value of £120,000, saving tens of thousands in SDLT compared to buying a completed property.

Standard stages: 1) Land purchase, 2) Foundations complete, 3) Wall plate level (walls to roof height), 4) Wind and watertight (roof on), 5) First fix (electrics/plumbing roughed in), 6) Second fix / plastering, 7) Completion. Each stage is inspected by the lender's surveyor before funds release.

Industry standard is to add 15-20% contingency on top of your build cost estimate. Unexpected costs are almost universal in self-builds: ground conditions, material price rises, design changes, weather delays. Without contingency, you risk running out of funds mid-build — potentially catastrophic.

Yes. The government's Help to Build scheme provides an equity loan of 5-20% of estimated land and build costs (up to £600,000 total) for self-builds. You need a minimum 5% deposit. The equity loan is interest-free for the first 5 years, then interest charges apply.

The self-build phase typically lasts 12-24 months. Once building complete, the self-build mortgage converts to a standard residential mortgage at a new rate. Most lenders allow 24-36 months for the build phase, with extensions available if needed.

Arrears mortgages release funds after each stage is inspected and verified — meaning you need cash or bridging finance to pay contractors upfront. Advance mortgages release funds before each stage begins. Advance is better for cash flow but fewer lenders offer it and rates may be slightly higher.

Yes. All self-build mortgage lenders require full planning permission to be in place before they will release any funds. You typically need outline planning permission before purchasing land, and full planning permission before the build begins.

You need specialist self-build insurance covering: site and materials insurance, public liability, employer's liability (if using workers directly), and structural warranty insurance. A 10-year structural warranty (like NHBC Buildmark or LABC Warranty) is usually required by the mortgage lender.

Living on site in a caravan is allowed under temporary permitted development rights for up to 12 months (renewable with planning consent). This saves rental costs during the build. Check with your local planning authority, as some areas restrict this.

Yes. Under HMRC's DIY Housebuilders Scheme (VAT431NB), you can reclaim VAT on building materials used in a new self-build home — this can be worth £20,000–£40,000 on a typical build. The reclaim must be submitted within 3 months of receiving your completion certificate.