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Good afternoon, has the freeholder responded to the S.42 yet?
The answer to your question would depend on the freeholder. It is possible to extend the lease term regardless how many years are remaining - it just means the premium for doing so would be far higher. However as long as the freeholder agrees then this is fine.
The costs involved would be the cost of the premium - i could not advise you of this because various factors are involved so you would need to obtain an independent valuation in respect of this - the cost of the valuation is anywhere between £300-£800. It is advisable to have this done so that you can counter offer what the freeholder puts forward should it be considered excessive.
Then it will be any legal fees - generally between £1-£2k.
The freeholder could refuse to extend the lease - but very very unlikely to refuse as it is in their interest to extend.
It is possible the extension could cost in the region of £250£100k.
The freeholder could take possession yes, but he would have to serve you notice of this and then provide two months for you to serve a counter notice to extend it. Unless the legal formalities are followed he cannot just take automatic possession.
There are no set guidelines other than the online calculator - which as you said cannot quote on less than 50 years. This is because it would be assessed on a bespoke basis dependent on numerous factors.
Remaining term, value of the property, market rate etc etc. There is no set maximum - which is why a valuation is strongly recommended to get an accurate idea of what it should cost. I cannot advise you of that.