Lease Extension Calculator: Inputs, Estimates and Limits
A calculator can establish a working range, but it cannot replace a leasehold valuation or predict statutory reform costs.
Before choosing a professional: credentials to check
Before instructing a solicitor, verify the individual and firm in the Solicitors Regulation Authority (SRA) Register. For valuations, verify any claimed RICS membership or regulated firm status through RICS. A membership badge is not proof by itself.
If you receive an informal offer, compare the proposed term, premium, ground rent and professional charges with the statutory route. An initial quote should explain what remains uncertain.
Your next step, made clearer
Avoid requesting a quote with half the facts. Prepare your lease, term and cost questions first.
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· England & Wales | October 2026
A calculator can establish a working range, but it cannot replace a leasehold valuation or predict statutory reform costs.
What an online calculation is estimating
An extension premium reflects the landlord's loss of future ground rent, its reversionary interest and, where applicable, marriage value. Inputs should reflect the specific lease, not a generic postcode average. A model may help with budgeting but should not be used as a legal offer.
Five inputs to collect before using a calculator
Obtain the commencement date and original term of the lease, current ground rent and scheduled increases, an informed estimate of the flat's value with a long lease, and the proposed legal route. A valuation professional may require more detail about intermediate interests or unusual clauses.
Why the range can be misleading
A small difference in remaining years, relativity assumptions or deferred land value can change a premium materially. A calculator does not ordinarily model a landlord's complete title, negotiation posture, a detailed ground-rent escalation clause or every tribunal scenario.
Compare the estimate with real costs
The premium is not the whole bill. Budget separately for a specialist valuer, your solicitor, potentially recoverable landlord costs under the current route and any dispute proceedings. Ask which assumptions the estimate treats as fixed and which are likely to be contested.
Reforms: do not price a future regime as if it is live
The 2024 Act outlines prospective valuation changes, including marriage value reform, but key provisions still require commencement. Verify the current law on the date of a statutory notice rather than relying on a calculator that silently applies anticipated reform.
Practical checklist
- Note lease commencement and expiry dates
- Record current rent and every increase
- Compare long-lease flat value assumptions
- Ask a valuer for a reasoned range
Common mistakes to avoid
A carefully documented decision is more useful than an assumption based on a search result. Check these risks before acting:
- Treating an online estimate as a fixed quote
- Forgetting landlord and professional costs
- Assuming uncommenced reform rules already apply
Questions to ask a qualified specialist
- Which jurisdiction and legal procedure govern my situation, and why?
- What facts, supporting documents or valuation evidence are missing?
- What deadlines, professional fees and potential adverse costs must be planned for?
- What alternative approach should I compare before proceeding?
This guide is general information for England and Wales, not individual legal advice. Some provisions of the Leasehold and Freehold Reform Act 2024 are not yet in force. No attorney-client relationship is created and no solicitor or lawyer referral service is active on this site.
References and further reading
Use these sources to verify the law and published procedures at the date you act; official guidance can be updated.
- lease-advice.org — official or institutional guidance
- lease-advice.org — official or institutional guidance
- lease-advice.org — official or institutional guidance
- lease-advice.org — official or institutional guidance
How to compare professional advisers
Ready to make a more informed decision?
Understand the lease, likely cost drivers and competing routes before committing to professional fees. Prepare a private, reusable enquiry summary and decide who to contact yourself.