We are in the process of trying the extend the lease on our flat. To get an idea of how much it would cost, I used some of the online lease extension calculators such as the one on www.lease-advice.org. These calculators came up with a range between £9000-£11000 for adding 90 years on. (The current lease has 78 years left, ground rent £100, and value of flat £250000).
However, when I try more 'sophisticated' calculators such as http://www.tenancy-agreements.co.uk/lease.php that let you enter the yield etc, they come out with significantly higher premiums. This is unless I enter yield values of 8% and above. Do the standard/simple calculators have a high built in yield value? Or is there some other reason for the discrepancy?
I ask because, following their valuation, the landlords have given us an offer of a 99 year lease with ground rent doubling every 25 years. But, this offer is 25% above what the calculators like the ones of lease-advise.org say they the premium should cost for adding 90 years with no ground rent.
Any advice on this would be much appreciated.