Answer: The freeholder will not necessarily say no, but you need to look at the terms of your lease. I have no doubt that your lease will say that any alterations to your flat will require the prior consent in writing of your landlord. However, it may also say that such consent must not be unreasonably withheld.
Your lease probably does not include the attic, making it part of the common parts, owned by the freeholder. If the freeholder is prepared to allow you to convert the attic so that it becomes part of your flat they are likely to require you to pay a premium, as the size of your flat will increase and the common parts will decrease. Also, your lease will need to be varied to show that the attic is now a part of your flat. Your share of the service charge is likely to increase.
In order to get your landlord's consent you are likely to be asked for detailed plans. The landlord will want to see that you have obtained any necessary planning and building regulation consents and listed building consent if appropriate. You will probably be asked to meet the landlord's legal costs in connection with the granting of consent and any necessary deed of variation.
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Fiona is a partner in the residential real estate team at Thring LLP (www.thrings.com).
These answers can only be a very brief commentary on the issues raised and should not be relied on as legal advice. No liability is accepted for such reliance. If you have similar issues, you should obtain advice from a solicitor.