Buying a property to use as an office

My partner and I are moving to the city soon (Burlington VT) and are going to be house hacking in a fairly small apt on one side of a duplex we are going to purchase. 

We currently have a home office in our house, but when we move, the apt will be a bit too small. I know a lot of people that can just work from their laptop at the kitchen table, but we have an energized 2 year old and I'm much more productive if I have a little space for an office. 

My partner is a massage therapist and rents an office to work on her clients. When we move, the amount she will have to pay for rent will most likely double to about $120/day. She may work 2 or 3 days a week, adding up to about $960 - $1440 per month. 

My thought is to buy a small property with a few bedrooms, maybe a small kitchen, within cycling distance of where we will live. She can use one room as her office and I can use one as mine. To make it a bit fun we could have little parties there sometimes or let relatives stay in one of the extra rooms.

I was wondering if there are any tax benefits to doing this? Or ways to look at this as an office hack? Maybe less benefits than a traditional rental property? Would it be legal to charge my partner rent? We aren't married but live together with our son. We don't share a bank account but technically don't differentiate between my money and hers.

 Technically this wouldn't be a home office right? Because it isn't my home, but I will own the building. There is also the idea to rent out a room as an office to another person as well.

Trying to find info on this question online has been tough, so I thought I'd ask all of you!

Thanks!