Building a GF would be a much better idea I think. Besides increasing the value of your property, you can also rent it out once it’s done. If that idea doesn’t appeal to you, you can also transform the GF into a home office, workshop or even a storage space. It costs cheaper to build too.

First check for council approval and second, you might want to reconsider the size of the GF you’re building. You do know you can build it up to 60 square meters though?

As to issues regarding tax, you’ll need to consult with an accountant on that.