Architect here. You need a permit to add a bathroom. I pretty sure depending on the size of the bathroom and your county (Mont. Co) you don't need a architects stamp. But that being said I think its a bad idea to build anything without a design or plan. You might want to work with a design build firm rather than a architect for a small project. I know Case Design has a design bath package with three different finish (price) levels. You might want to check into this.

http://www.casedesign.com/our-work/what-to-expect/baths/bath-design-packages

I would just leave it, because it will cost you anywhere from 3-5k for drawings and the permit fees. If you are type of person goes by the book and don't want any future hassle, then go ahead spend the money.

We had to draw and pay for permits because it was almost I g gutting the entire house.

Oh and make sure you hire a reputable contractor, out project was 3 months late.

I am planning a minor renovation of my DC townhouse's second floor (adding a bathroom and some built-in closets). Do I need an architect or will a contractor suffice in terms of assembling the permit review package? Do contractors typically do drawings? The permit application has space to list both an architect and contractor.
thanks!