Let to Buy Mortgage Calculator
Use this calculator to estimate how much you may be able to borrow when remortgaging your current home onto a buy-to-let basis, then releasing equity to support your next purchase.
This is an educational estimate only. Actual let-to-buy affordability depends on lender policy, personal income, credit profile, property type, tax, and underwriting.
What is a let to buy mortgage?
A let to buy setup is used when you want to move into a new home but keep your current property and rent it out. Instead of selling your existing home, you remortgage it as a buy-to-let property and release equity for the deposit on your next residential purchase.
In simple terms, you are running two mortgage arrangements:
- Mortgage 1: A buy-to-let mortgage on your current home.
- Mortgage 2: A residential mortgage on the home you plan to live in.
How this calculator works
This tool estimates your potential borrowing on the let-to-buy property using two common lender constraints:
- Rental stress test (ICR-based): Ensures expected rent covers interest payments at a stressed rate.
- Maximum LTV cap: Limits borrowing to a fixed percentage of the property value (for example, 75%).
Your estimated maximum let-to-buy mortgage is the lower of those two values. From there, we subtract your existing mortgage balance and estimated costs to show likely net equity release.
Core formula used
- Max Loan by Rent = Annual Rent ÷ (ICR × Stress Rate)
- Max Loan by LTV = Property Value × Max LTV
- Estimated Max Loan = lower of the two figures above
- Net Equity Available = (Estimated Max Loan − Outstanding Mortgage) − Costs
Why your result might differ from lender offers
Even with a strong estimate, lenders can still vary significantly. Some lenders apply different stress rates for basic-rate and higher-rate taxpayers, others model affordability based on product type, and some include minimum income requirements for landlords.
Common reasons for lower final borrowing
- Conservative valuation from lender surveyor
- Property type restrictions (ex-local authority, non-standard construction, small flats)
- Personal credit or debt-to-income profile
- Portfolio landlord rules if you already own multiple rental properties
- Fees added to the loan reducing effective headroom
How to improve let-to-buy affordability
- Increase expected rent by improving presentation or adding amenities before valuation.
- Reduce outstanding mortgage balance where possible.
- Compare lenders with different ICR and stress test assumptions.
- Keep clean credit usage before application.
- Work with a whole-of-market broker familiar with let-to-buy cases.
Worked example
Suppose your current home is worth £425,000, your mortgage balance is £210,000, and expected rent is £1,750 per month. If your lender uses 145% ICR, 5.5% stress rate, and 75% LTV:
- Max by rent might be around £263,000
- Max by LTV would be £318,750
- Estimated max mortgage becomes £263,000 (rent is the limiting factor)
- Gross releasable equity = £53,000
- After £3,000 costs, net available is around £50,000
That net figure can then be used toward your deposit and moving costs on the new residential purchase.
Before you proceed
Let-to-buy can be powerful but complex. Make sure you account for landlord responsibilities, void periods, maintenance, insurance, tax implications, and rate risk. If the numbers are tight, stress-test your own cash flow beyond lender minimums.
Use this calculator as a planning tool, then validate everything with a qualified mortgage broker and tax adviser before making commitments.