Hi
The garage if it is in a good state of repair, is an asset as it provides additional storage space or use as a workshop etc. Not everyone wants a big garden these days as they don't have the time to maintain it.
If I was considering this I would ask a local estate agent around to give me a free valuation. You could tell a white lie, that you were thinking of possibly putting it on the market at some point. This would enable you to ask a question about the local market and the relative value of Garage v larger garden.
Huckster