There are two kinds of residential additions: horizontal and vertical. The type you choose is often dictated by the purpose of the addition—kitchen additions are invariably built horizontally, for example—but it’s not always that simple. Before making a final decision on whether to build horizontally or vertically, consider the following information about the advantages and disadvantages of each:
Building out (horizontal)
The vast majority of residential additions (kitchens, family rooms) involve increasing the footprint of the ground floor level of the building.
What’s involved: Generally, your contractor will bring in a backhoe to dig up the yard in the area where the addition will sit, install a new foundation or slab, and then construct the walls and roof of the addition before opening up the existing exterior wall and linking the new and old spaces.
Advantages: Building horizontally typically involves the least disruption to the existing space, because the new space isn’t supported over the existing structural framing or foundation. Also, if you’re only creating a small addition, you may be able to do a bump out and avoid any foundation work whatsoever.
Disadvantages: Building horizontally means losing some of your yard, and it might even require a city zoning variance if you’re within the legal property line “setback” (usually seven feet from the neighbor’s property). You could also face limitations from the city’s “Floor-Area-Ratio” rating, which dictates the proportion to which a lot can be built.
Building up (vertical)
There are many ways additions can be built without expanding the footprint of the house: You can add another story onto a home; expand an existing top floor by installing large dormers in a pitched roof to gain useful living space; or add living space above an existing garage, porch, sunroom or other one-story wing.
What’s involved: Although there’s no need to give up a portion of the yard to a new foundation for the addition, your contractor will likely have to expose and strengthen the existing foundation and wall structure under the new space to ensure it can support the added weight. Adding a room means adding a significant load to all the elements underneath it.
Pros: You won’t lose any yard space or run into zoning restrictions/Floor-Area-Ratio limits.
Cons: Many cities limit the allowable height for houses, which can be an issue when building vertically. If you’re adding an additional story, you’ll also need to account for a stairwell, which can easily take up 80 to 120 square feet of living space. Also, your contractor will likely have to tear apart the walls and ceilings in the space below to strengthen the structural supports and feed in the electrical, plumbing and heating lines.
