The interest and mortgage on the whole house is deductible regardless of whether or not it's business property. That will help on my fiance's taxes in general since he rented previously which is NOT tax deductible.
Now, if you use a part of that house as business space, that business space is deductible on the business taxes. Since the house we own is bigger than the apartment he had, that will now be a bigger percentage. The things that are tax deductible on that space also include a percentage of the utilities used in that space... So some percentage the accountant derives.
So I am saying that that is some kind of additional deduction. It's going to be confusing, though, because I also occupy the house, have 50% ownership, and also report taxes. This year, we are not married, so who knows how that gets divided up. Next year, we have the option of filing together as married. Or we could file separately as single, depending on which gives us the best advantage. These are things for the accountant to crunch around. It sounds messy to me!