Buildings are not tax deductible, and therefore garden offices or extensions for the purposes of working from home cannot be claimed as a business expense.
No relief will be provided for work spaces within domestic settings, such as home-offices
HMRC Guidance, Allowances for Structures and Buildings, 2018
Although buildings are exempt, you can claim for fixtures and fittings, such as desks, chairs, storage etc which are used exclusively for the business.