@Jeremy D. To add what @Andrew Spell mentioned if you want to separate a home into two units you need to find an RF-1 zoned lot which allows this by right. This is the most common zone to do this in DC. Other zones are along major streets where they allow higher density. If you want to add more than two units in a RF-1 zone you can go through the BZA (board of zoning appeal) and ask for three or more units and file for a special exception. This petition may be denied and takes around 4-6 months to complete.
There are a few big difference with an ADU and converting a single family house into two dwelling units. Here are a few.
1. ADU is under the same owner. If you sell the house the ADU is included.
2. ADU is on the same utilities as the main portion of the house.
3. An ADU can only be 35% of the main house. There is also a max SF but I don't remember it. I think it is 700 SF.
4. Two dwellings allow it to be legally separated into two distinct homes. Each homes requires their own utilities and fire proofing between units.
5. The new unit will need fire sprinklers. Unless you do a third story pop up which will then make the existing floors require fire spot Klerk as well.
6. Both units can be the same size.
7. The two dwellings can go through the process of a condo conversion and is expensive and a lengthy process. People do this so that they can rent out both units or even sell one or both. Most of my developer clients do this.
8. Creating two units is expensive and usually costs around $125k - $185k. There are many factors but this is what I see for my clients.
To easily find a property and their zoning information go to https://propertyquest.dc.gov/. Type in the address and you will get a lot of information.
If you want more in-depth information about a property you can go to https://scout.dcra.dc.gov/login
You can also find a list of vacant and blighted homes. Go to the DCRA website and search for it. They put out the list every quarter and you might be able to find some deals because those owners are paying an arm and leg in taxes once the property is classified with those distinctions. However, these homes require a lot of work. I walked through a blighted house last week and my foot fell through the floor.
Good luck.