Estimate the premium in seconds — the loss of ground rent, the reversion, and the marriage value once you drop below 80 years. No sign-up. An estimate, not a formal valuation.
Guess the value high and the premium looks scary; guess it low and you under-budget. PropertyPredator pulls the real sold comps for your postcode (£/ft² and £/m²) so you start from the true value — and tells you whether the deal still stacks after the extension cost.
Under 80 years, "marriage value" kicks in — the extra value created by extending is split 50/50 with the freeholder, so the premium jumps sharply. Extending before 80 years is usually far cheaper. This calculator flags it.
It uses the recognised valuation method (term, reversion and marriage value with a relativity curve), so it's a sensible ballpark — but real premiums come from a RICS surveyor's valuation and negotiation with the freeholder. Treat this as a budgeting guide, not a quote.
This estimates the premium for a flat lease extension. Leasehold houses follow a different process. Always take professional advice for your specific lease.