If it were the shed option - that can easily be a business asset, claim VAT on all and claim CAs where relevant on fittings.
If teh garage option I would be very wary of VAT reclaim on any of the structural works.Not just the VAT angle but other obvious future CGT options.
The interior of the office (fittings etc) can be classified seperately and claimed as appropriate for CAs and VAT (as if it were a larger business carrying out a refit of a new leased office space) - but again on these be aware of tfr of ownership when the business ceases.
Not sure if others agree but this would be my starting approach.
AND the over-riding factor would be - after deducting a private element for teh VAT on the structural work - how much VAT are we actually talking about - I suspect - not a lot as garage conversions are not that expensive .....