We have found that third-story pop-ups on rowhouses are becoming more popular in Washington DC because the influx of people moving to the city and the density of neighborhoods means rowhouse owners either have to go up or back to gain the additional square footage they desire. Homeowners don’t want to move to gain additional space because they like their neighborhood or their kid’s schools. Most want a third-floor addition for a new master suite. However, if you’re looking for a larger family room, a third-story addition doesn’t make sense. Some turn to third story additions because zoning rules may limit the size of a rear addition. This applies to rowhouses and semi-detached houses. General rules for a rear addition are:

• With zoning you are limited to a 10-foot addition on the back or no further than 10-feet past your adjacent neighbor’s rear elevation.
• The Floor Area Ratio of the zone where your home is located can limit the rear addition size to 40%, 50% or 60% of your total lot size. A third-story addition does not affect FAR. In general, as a right, you can add a third story to two-story rowhouses. There is a 40-feet height limit. Structurally, we have found that almost all two-story rowhouses can carry the load of a third floor, but to confirm this, we will (and DCRA requires) dig test pits to verify that the existing footings, foundations, and soil conditions will support a third floor.