There are 2 issues here:
1. I am assuming that the house that the garage is part of is personally owned by your client rather than by the business. Capital allowances therefore does not arise at all in terms of the cost of improvement to the garage that will add value to the property. However plant and equipment or fixtures and fittings that will be used for the business may qualify for capital allowances.
2. Even if the business owned the house, the cost of making improvement to the garage, though capital expenditure, does not qualify for capital allowance as you can't claim capital allowance for buildings unless they are in a 'disadvantaged area' (see https://www.gov.uk/guidance/business-premises-renovation-allowance-bpra).