Lease extension premiums (the part payable to the freeholder for the grant of the new lease) are complicated. They are not something that an estate agent or Zoopla can assist with.
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We link to a simple calculator above however there are many elements are play when calculating the premium payable under the Leasehold Reform, Housing and Urban Development Act 1993, including marriage value in some cases (for leases with less than 80 years) and capitalising ground rents including doubling ground rents, RPI ground rents and ground rents linked to a percentage of the sale price of you property. There are also Tribunal decisions on things such as roof space development value and other nice areas.
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There is an entire profession dedicated to valuing lease extension premiums and negotiating them. More information can be found on the Lease Advice website, an independent source of information.

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Are there other costs in addition to the new lease premium?
The calculator is a useful tool for guiding you on the likely premium however it is not suitable in all circumstances, for example the calculator cannot take account of rising ground rents (which are very common). Only an experienced leasehold valuer can provide a reliable valuation with a range of the likely premium payable to the freeholder.
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Once you have established the likely premium, you will need to budget for the following costs:
Your own legal costs of perhaps £950 - £2,000 depending on the solicitors you appoint
You own valuation costs of typically between £500-£800
Negotiation fee for your valuer to negotiate on your behalf
Freeholders legal costs of typically £1,600 plus VAT but this can vary considerably depending on the firm appointed by your freeholder (they are entitled to recover 'reasonable legal costs' however the FTT has upheld landlord legal costs of £3,000 plus VAT in cases we have been involved with, and so this leaseholder protection isn't entirely useful)
Freeholders valuation fee of around £500-£800, note that you should under no circumstances pay your landlords negotiation costs, as these are not recoverable under section 60 of the Leasehold Reform, Housing and Urban Development Act 1993.