With last week's post on affordable housing I was wondering: if we did build out all the Somerville lots zoned for four stories (MR4) to be seven stories of affordable housing, how much housing would that be? I count 254 lots zoned MR4 with a total area of 1.8M sqft. Built out at seven stories and an average of 80% coverage that's 10M sqft of housing. Figure 10% for stairs, elevators, and hallways, and it's really 9M for units. Let's say you build:

bedrooms	portion of construction	unit size
2br	25%	850 sqft
3br	25%	1000 sqft
4br	25%	1100 sqft
5br	25%	1200 sqft

Then building all of this would give you:

bedrooms	number of units
2br	2230
3br	2230
4br	2230
5br	2230

These properties currently comprise just under 1k units, so this is 7.9k new units of affordable housing and 1k units converted to affordable housing. This would bring Somerville from ~35k units to ~43k.