My understanding is that you will be encouraged by the Council to use (and pay for) a lawyer and a surveyor, the latter to opine on the value of the proposed lease extension. You should also budget for paying the Council’s legal / surveying costs. So, if the actual cost of the lease extension is say £2500 (guesstimate!), there might be further costs of £5-7K when all the extras are totted up. You might be able to “do” the valuation yourself, and indeed the legals I suppose, but how would you know you are going about it the right way? Hence the reason why most people use professionals. I’m not sure if you can wriggle out of paying the Council’s costs – worth researching I would have thought.