Hello
I think this is similar to the problem that anyone who is self-employed and working at home faces (I work at home, self-employed, but not in nails at the mo!). If you have a room (whether in your house or garden) that is solely used for business, then it will be treated as business premises and this brings with it a whole lot of problems not only from an insurance point of view, but from a business rates and capital gains tax (profit you make when you sell the property) point of view (and possibly planning too!).
The best solution that I use is to ensure the room has (sort of) dual use - e.g. if you put a sofa in your garden room, you could say that you use it for your own summer house (for personal use). Or put a sofa bed in it and say it's your guest room. Or put a TV in there and say it's the kids' TV room (not that kids and manicure stuff go well together, but you get the gist!). I'm not sure if anyone would ever come and check but in my experience, it's never good to say that you are using a room solely for business in your house/garden.
Your home insurer may then be more amenable and it should not be uncommon for them to cover this building as it is mixed use, given the number of people who work from home these days (in all industries). Your business contents should be covered by a mobile / home salon policy?
Just my views!
