How this extending a lease calculator works
A lease extension premium is usually made up of three moving parts: the landlord’s loss of ground rent, the landlord’s delayed right to get the property back (reversion), and in many cases marriage value. This calculator combines those components into a practical estimate so you can plan your next step.
It follows the common structure used in England and Wales lease extension discussions. The output is not a substitute for a RICS valuation or legal advice, but it can help you understand whether your likely premium is in the low tens of thousands or significantly more.
Inputs you should understand before using it
1) Long lease market value
This is the estimated value of the flat with a long lease and peppercorn ground rent. If you only have an estate agent range, use the mid-point and then test a higher/lower figure to see sensitivity.
2) Ground rent and growth
If your ground rent is fixed, keep growth at 0%. If your lease has escalating clauses, add an approximate annual growth rate. The calculator discounts these payments into today’s value using your capitalisation rate.
3) Years remaining
This is often the most important variable. As the lease gets shorter, the premium rises faster. The jump in cost can be especially noticeable once the lease falls below 80 years.
4) Capitalisation and deferment rates
- Capitalisation rate discounts future ground rent income into present value.
- Deferment rate discounts the landlord’s future reversion value.
Small changes in these rates can materially change results, so always test a range and compare outcomes.
Why the 80-year threshold matters
In many lease extension cases, marriage value becomes payable when the unexpired term is below 80 years. In simplified terms, marriage value reflects part of the uplift in value after extension. The calculator applies a 50% share of that uplift (when positive) once your lease length is below 80.
How to use the estimate in real life
- Run a base case using your best estimate of market value.
- Run a conservative case with higher value and lower relativity.
- Use the range to set budget expectations for valuation, legal costs, and negotiations.
- Take the figures to a specialist valuer and solicitor before serving formal notice.
Important limitations
This model is intentionally simplified. Real-world premiums can vary due to lease clauses, local evidence, relativity assumptions, Act route, tribunal outcomes, and negotiation strategy. Treat this as a planning tool, not a final number for legal service.
Quick strategy tips for leaseholders
- Do not wait too long once your lease approaches 80 years.
- Collect title documents and lease wording early.
- Choose professionals with direct lease extension experience.
- Budget for both premium and professional fees.