DC is a great place for a bed and breakfast. I have noticed some mixed into residential areas, zoning can be different from block to block.
There are two important departments you'll need to contact about basic information to get started. The fist one is the DC Department of Regulatory Affairs (DCRA) to obtain a license. http://dcra.dc.gov/DC/DCRA
Then you will need to contact the DC Office of Zoning, I've attached this site as well for you (http://dcoz.dc.gov/info/map.shtm). Please call me if you have any questions.
I know of some great places in DC that may be a good fit for your business needs, and would love to discuss it further with you.
That is going to depend on what the zoning for where the property is located is zoned for. If it's zoned just for residential, then most likely the answer is no. If it's zoned mixed use, then you may be able to. You will need to check with the zoning board for the city to find out what is allowed.
There are several great bed and breakfast through out Washington DC neighborhoods. One being in the Bloomingdale section of Ledroit Park. I'm sure with the correct license and permits etc... you will be able to do the same.