Residential house used as an office - Tax write off

[Jason Andrews]

I purchased the small 2bd/1bth house with garage next door to our home to use as an office/business space possibly. My wife has a small clothing business (traveling sales-doesn't sell from home) and I have started getting into rent houses. So far we have been using a 20x20 room in house as an office that we write off for business use on our taxes. Could I write off the complete other house that I purchased for both of our businesses? Or does this house need to be deemed commercial property (which I don't and probably can't do in a residential area)? 

We are located in Texas.

Thanks