In Arlington Heights, the fifty-foot lot decides where your next room goes.

Not too narrow — narrow in one direction. Across the width, two side yards and usually a drive come off before the house does, so there is rarely room to build sideways. Along the depth there usually is: the median lot here runs about 130 feet front to back. So the useful question is not whether to add but where, and the lot, the trees and the drive answer it before a floor plan does. We measure the lot first.

From its parcel, not from the street. Historic Hillcrest covers a stretch of Tremont Avenue and is drawn on the city’s zoning map, but nothing at the curb marks where it starts or stops. Inside it, exterior work needs a Certificate of Appropriateness before the permit. Outside it, which is most of Arlington Heights, the front of the house follows the ordinary residential permit path. Deed restrictions are their own question, apart from city zoning, and a block with no overlay can still carry one, so they are worth checking for any address.

About half of the neighborhood is A-5 One-Family. New lots there must be at least 5,000 sq ft and 50 ft wide; a house may cover up to 50% of its lot and rise to 35 ft; side yards are 5 ft; and the front yard is 20 feet on a local street or 15 on a limited local street. About a quarter is B Two-Family, where a two-family house is allowed and two detached homes need a larger lot. Then one more rule changes the arithmetic: where at least 40% of a block face is built, Fort Worth sets your front yard by the houses already standing on it instead of by the district number, up to a 50-foot cap (§ 6.101(d)(1)c). The front-yard number in the table may not be the one for your house; the parcel answers it.

Price both before you choose. On a lot whose width is already spoken for, the remodel usually means building into the depth of the lot, and that has a real cost. Our agents will price your house as it stands and show you what a larger one here costs to buy; our drafting team and builders will price the change itself. Because the designers, builders and agents work side by side, both numbers come from people who understand the other half. Sometimes the right house is already for sale a few streets over, and we will tell you so.

Rock sits close to the surface across the largest part of the neighborhood, about a foot down in the most widespread soil reading. What that does to a project is mostly a matter of cost rather than of whether it can be built: excavation, plumbing trenches and piers all have to get through it, while the clay movement North Texas foundations are famous for matters less here. Ground changes from one block to the next.

Usually, and the lots are the right shape for it: deep rather than wide. What the back building can be depends on the parcel’s zoning. In A-5 it is an accessory dwelling, planned around family, guests or an office rather than as a separate home; on land zoned B Two-Family a second home is allowed if the lot is big enough. Either way the driveway has to reach it unless an alley does, it may not be taller than the main house, and both side yards come out of the width. We have built one on these streets, and the garage-apartment page for this neighborhood goes further.