For a half bathroom or powder room addition in Washington DC, here are some basic minimum dimensions you should consider:

1. Overall Size: Aim for a minimum of 20 square feet of floor space, though 30 square feet is more comfortable.
2. Width: At least 4 feet in width for a comfortable layout.
3. Length: Minimum of 5 feet in length to accommodate essential fixtures.
4. Toilet: Clearance around the toilet should be at least 15 inches from the centerline of the toilet to any side wall or obstruction.
5. Sink: Allow for about 30 inches of width for a standard sink and countertop.
6. Clearances: Ensure there’s adequate space around the door swing and fixtures to allow for comfortable use.

These dimensions ensure that the space is functional and meets building code requirements. Always check with local building codes and consult with a professional architect or contractor for specific requirements and to optimize your layout for usability and comfort.