Buy-to-Let Calculator UK

New properties enter the portfolio at their end value after refurbishment, with refurbishment completed in the purchase month. Both model purchase price and end value grow with annual house-price growth before later purchases; owned properties then grow from their completed value. Refurbishment is paid once per purchase and does not itself determine the uplift. Mortgages and SDLT are based on purchase price. Existing properties retain their entered current values. Construction delays and refinancing are not modelled.

Calculations run monthly: contributions, annual rent adjustment, rent less vacancy, costs, mortgage payments, value growth, retained surplus, then purchases. Month zero contains the existing portfolio and any immediately affordable new purchase. A new purchase first earns rent the following month. Property and fixed-cost growth use equivalent compound monthly rates. Percentage costs track rent.

Owned properties start at current value and outstanding debt. Historical purchase prices are reference information; no deposit, SDLT or acquisition cost is charged again. Starting cash is separate from equity, and personal cash contributed counts only cash added from the forecast start. Existing interest-only mortgages pay interest and retain their balance; no maturity balloon or refinancing is assumed. Existing repayment mortgages amortise over the entered remaining term and payments stop once repaid. Older scenarios retain their entered payment until a mortgage type is selected; insufficient entered payments add unpaid interest to debt. Payments are capped at debt plus interest, and interest rates are held constant beyond fixed-rate end dates.

New mortgage payments follow the selected repayment basis and term. Interest-only principal falls due in cash at term expiry. Equity cannot fund purchases without a sale or refinance; neither is modelled. The default investor SDLT is included on every new acquisition; future tax bands are held constant. Historical SDLT before April 2025, LBTT and LTT are not supported.

Income/corporation tax, capital gains tax, lending affordability, refinancing, early repayment charges and other exit fees are excluded. Ownership is recorded but does not apply personal or company income tax rules. All growth assumptions are illustrative. Calculations retain decimal precision; displayed cash rounds to pennies and the final SDLT charge rounds down to whole pounds.