I looked into this a few years ago with a view to converting the garage.
1. If you work in a defined office area as opposed to occasionally working on the Kitchen (!) Table, you may become liable for rates. Rates arise from the carrying on of a commercial undertaking apart from the situation described above.
2. If you decide to write off the cost if a premises (including the VAT) and you later sell the premises as part of a house, I understand there is a "Hit" to be taken at that point in terms of the VAT not paid and the capital write off.
I didn't look into it further as I didn't proceed and my information above may be incorrect, but it shows that you don't get something for nothing.
ONQ.