Lease extension offer calculation

[Theyarv]

Hi,

I am buying a flat and i need to get the lease extended as it has 80.5 years left when we complete (current owners will start the section 42 notice and hand that over so not to worry:) )
We (buyers) have to ask the vendors to put a price in the notice (since we will be paying it).


It goes to 80 years in september
Lease extension will be 90 years
Ground rent i believe is a peppercorn
future market value - not idea (flat is islington, London if that makes a diff)
yield - no idea how to calculate this
Marriage value - 0 as it will be above 80 years

I was wondering if anyone could give me an approximate price that i realistically might hope to acquire the lease extension at. If it is a price that is unrealistic then the notice could be void and we will have to wait 2 years to extend it, meaning a horrible marriage value.

If it's not too much bother maybe could i get a more conservative value as well (say £10 ground rent and tweaking a couple of the other variables) to give me a realistic upper limit of what an offer might look like.

Much appreciated!

[Theyarv]

Just to update

I have used a number of calculators and have come up with a figure of between 5175 and 5260 on a GR of £10

and between 6160 approx on GR of £50

[sgclacy]

You need to state the value of the property and the ground rent

However based on the info you have given the mount would be

Take the value of the flat and divide it's value by 50.8

Take the ground rent and muliply it by 17

The sum of the two is the premium

This is a very rough and ready calculation but there is an enormous lack of detail in your question