In short: This free calculator gives an indicative premium for a statutory lease extension on a flat in England or Wales under the Leasehold Reform, Housing and Urban Development Act 1993, which is the law in force today. Enter the flat's value with a long lease, the years left on your lease and the ground rent to see an estimated range. The calculator was built by Aston James Associates, RICS chartered surveyors and Registered Valuers who carry out lease extension valuations across London, the South East and Yorkshire.
Last reviewed: 16 October 2026 by Patrick Oliver BSc (Hons) MRICS, RICS Registered Valuer.
How is a lease extension premium worked out?
Under the 1993 Act, the premium you pay for a 90-year extension at a peppercorn ground rent is made up of three parts:
- Loss of ground rent. The landlord gives up the ground rent for the rest of your lease. Its value is the rent capitalised over the remaining years, usually at 5% to 7%.
- Loss of reversion. The landlord would normally get the flat back when the lease ends. After the extension it has to wait a further 90 years. The value of that delay is worked out using a deferment rate of 5%, set by the Sportelli case.
- Marriage value. Where the lease has 80 years or less left, extending it raises the flat's value. The landlord is entitled to half of that increase.
Worked example. A flat worth £400,000 with a long lease has 70 years left and a fixed ground rent of £250 a year. With the current lease it is worth £340,000. The ground rent is worth about £4,096 (capitalised at 6%). The landlord's interest falls in value by about £17,211 in total. Marriage value is about £42,789, of which the landlord takes half, £21,395. The premium is therefore about £38,605. Most of the cost comes from marriage value, which is why the 80-year point matters so much.
Why does the 80-year threshold matter?
If your lease has more than 80 years left, no marriage value is payable and the premium is mostly the value of the ground rent and a small reversion. Once the lease falls to 80 years or below, marriage value starts to apply and grows each year, so the premium can jump significantly. If your lease is approaching 80 years, serving a formal notice (a section 42 notice) before it drops below 80 fixes the valuation date. See our guide to section 42 notices.
What about leasehold reform? Should I wait?
The Leasehold and Freehold Reform Act 2024 includes the abolition of marriage value and standard 990-year lease extensions, under a new valuation method using rates set by the government. However, those valuation provisions are not yet in force and no commencement date has been announced. Lease extensions today are still priced under the 1993 Act, which is how this calculator works. If your lease is below 80 years, waiting means it keeps getting shorter, and marriage value keeps rising, while the reform date remains unknown. We will update this calculator when the new method comes into force.
What does the calculator assume?
- A statutory 90-year extension at a peppercorn rent under the 1993 Act.
- Freehold value taken as the long-lease value divided by 0.99.
- Deferment rate of 5%. Ground rent capitalised at 6% for the central estimate and 5% to 7% for the range.
- Where you do not enter a value with your current lease, relativity is taken as the average of the Savills 2015 unenfranchiseable and Gerald Eve 2016 graphs, as in Deritend v Treskonova, with a band of 2 percentage points either side for the range.
- No allowance for compensation, intermediate (head) leases, tenant's improvements or either side's legal and valuation costs.
When do I need a formal valuation?
You need a formal valuation before serving a section 42 notice, before negotiating with your landlord, and if the matter goes to the First-tier Tribunal. A formal valuation looks at the evidence for your particular flat, which can move the premium well away from a standard estimate. Our lease extension valuations are carried out and signed by an RICS Registered Valuer. If your whole building is considering buying the freehold together, see our guide to collective enfranchisement.
Who is behind this calculator?
The calculator was prepared by Patrick Oliver BSc (Hons) MRICS, RICS Registered Valuer (RICS membership number 0858120), who specialises in lease extension and collective enfranchisement valuations under the 1993 Act. Aston James Associates, working in association with Earlswood Valuations Ltd, is regulated by RICS (firm number 881204). See our credentials and regulation.
Important: This is an indicative estimate only, based on standard assumptions and the figures you enter. It is not a valuation, is not advice, and must not be relied on for any transaction or statutory notice. A formal valuation by an RICS Registered Valuer is required.
