Skip to content
    First Rung Now
    First Rung Now
    UK Mortgage Guides
    Speak to a Vetted Broker

    UK Mortgage Guide

    Mortgage Buyout Calculator UK: How to Work Out the Real Cost

    Working out what it really costs to buy out a partner from a joint mortgage is a maths problem with several layers. There's the equity split, then the affordability test on your sole income, then stamp duty, legal fees and lender criteria — all of which interact. This guide walks through each step the way a UK mortgage broker would, with worked examples and the assumptions you need to make before any conversation with a lender.

    First Rung Now Editorial Updated 15 June 2026 7 min read

    The five-step buyout calculation

    1. Step 1: Current market value. Get 2–3 valuations from local estate agents. For an accurate figure, commission a RICS Homebuyer Report (£400–£700).
    2. Step 2: Outstanding mortgage balance. Ask the lender for a redemption figure including any early repayment charges and exit fees.
    3. Step 3: Calculate equity. Value − mortgage = equity. Don't forget to deduct any other secured borrowing (second charges, Help to Buy equity loan).
    4. Step 4: Apply the ownership split. 50/50 is the default for joint tenants. Tenants in common can hold any agreed split — check the deed of trust if one exists.
    5. Step 5: Add the buyout payment to the existing mortgage. This is the new loan you need a lender to approve.

    Worked example: standard 50/50 split

    • Property value: £400,000
    • Outstanding mortgage: £250,000
    • Equity: £150,000
    • 50% share to leaving party: £75,000
    • New mortgage required: £250,000 + £75,000 = £325,000
    • New LTV: 81% — within mainstream lender appetite, but pricing reflects the higher band.

    Worked example: uneven split with second charge

    • Property value: £550,000
    • First mortgage: £280,000
    • Second charge (home improvement loan): £20,000
    • Total secured debt: £300,000
    • Net equity: £250,000
    • Tenants in common 60/40 — leaving party owns 40%: £100,000
    • New mortgage = £280,000 + £20,000 (clearing second) + £100,000 = £400,000
    • New LTV: 73% — sharper pricing but higher absolute affordability test.

    The affordability check that often blocks the buyout

    Calculating the loan you need is the easy bit. Lenders then apply income multiples (typically 4.5×–5.5× sole income) and detailed expenditure analysis. On the £325,000 example above, you'd typically need a single income of roughly £60,000–£72,000 to be approved, depending on outgoings, credit profile and lender. If joint household income was £100,000, sole-name affordability often falls short. Three common workarounds:

    • Longer term. Extending from 20 to 30 or 35 years reduces the monthly payment and increases the maximum loan.
    • Joint Borrower Sole Proprietor (JBSP). A parent's or sibling's income is added to the affordability calculation, but they don't go on the title — meaning no second-property stamp duty.
    • Specialist lenders with higher multiples. Some lenders go to 5.5×–6× income for professionals on higher salaries.

    Stamp duty on a mortgage buyout

    Stamp Duty Land Tax (SDLT) is triggered by "chargeable consideration" — the buyout payment plus the share of debt assumed. If the total exceeds the SDLT threshold (£40,000 under the additional dwellings rules; £250,000 main residential threshold), SDLT applies. Critical exemptions:

    • Divorce or dissolution of civil partnership under a court order: No SDLT.
    • Transfer between spouses or civil partners not in formal proceedings: SDLT can apply on the consideration.
    • Inheritance: No SDLT on the transfer itself.

    Always get specific legal advice. Mistakes are expensive to undo.

    Costs to budget alongside the buyout

    • Conveyancing (transfer of equity): £500–£1,500 for each party.
    • Mortgage product/arrangement fee: £0–£1,995 depending on product.
    • Valuation fee: Often free with the new lender.
    • Broker fee: £0–£1,500 if you use one.
    • Early Repayment Charge (ERC): Check the existing deal — ERCs of 1%–5% of the balance can apply if you remortgage during a fixed term.
    • Land Registry fee: Scaled by property value, typically £20–£455.

    Process and timeline

    1. Agree the principle in writing — value, equity split, buyout figure.
    2. Get a Decision in Principle from a lender in sole name (or with new joint borrower).
    3. Both parties instruct solicitors.
    4. Submit full mortgage application — typical offer in 2–4 weeks.
    5. Conveyancing runs in parallel — 6–10 weeks total is common.
    6. Completion: leaving party is paid, mortgage drawn down, Land Registry updated.

    Pros

    • Lets one party keep the family home — major for children and stability.
    • Cleaner separation than continuing co-ownership.
    • Opportunity to remortgage to a better deal at the same time.
    • Court-order transfers are SDLT-exempt.
    • Tax-efficient when planned with a solicitor and broker together.

    Cons

    • Sole-name affordability is often significantly lower than joint.
    • ERCs on existing deal can add thousands.
    • Higher LTV after the buyout means higher rates.
    • Legal and conveyancing fees double up (each party needs representation).
    • Property valuations can swing the equity calculation by tens of thousands.

    Frequently asked questions