Hello and thank you for your question. My name is Lisa. I am a qualified solicitor and would be happy to assist.
I understand that you have a long leasehold interest in your property and that you have approached the Landlord for the grant of a deed of variation to your lease, to enable you to use your garage as part of your living space.
Assuming that a deed of variation is required to enable you to do this (in other words, that the lease currently prohibits use of the garage as living space and must therefore be varied), there is unfortunately nothing that requires the landlord to enter into a variation, or limits the premium he may charge for doing so. It is entirely within his power to simply refuse altogether, or to request a very high payment to sign up to the variation.
I appreciate this probably isn't the news that you were hoping for and am sorry to be the bearer of bad news. Hopefully, though, this at least assists to clarify the legal position.
Please do reply below if you have any further questions or would like any aspect of my response clarified. If my comments are of assistance, I would be grateful if you would confirm by accepting and rating this response. This can be done by selecting one of the five stars located on the top right of the question page and enables me to gain credit for the time taken in responding to your query. Please note that rating the response will not close this question - you will still be able to post below with any further queries or comments.