Building a multistory addition solves the problem of a small lot by adding the maximum amount of living space within available setback limits. It also allows you to capture a majestic view, preserve open space in your yard, or use extra space over the garage.
If you are adding a completely new two-story structure, most of the work will not disrupt the existing home. But building on top of living space can be a major challenge to both occupants and builder. The roof will be partially or totally removed, and extensive work on the foundation and first floor may be needed before work on the addition begins. Such a project requires thorough planning, crisp coordination, and the determination to keep it moving forward.
Multistory additions involve the basic functional, structural, and aesthetic design issues of any living space, but with some unique problems.
Stairs. Stairways take up a surprising amount of space (50 to 60 square feet of downstairs floor area and another 35 to 45 square feet of upstairs area) and should therefore be as carefully designed as any room.
In a two-story addition, stairs can be placed within the new space or, if your home already has two stories, it may be possible to use existing stairs. On the other hand, locating stairs in the present living space is difficult. Sometimes they can be placed in closets or underused rooms, such as a laundry room or office that's scheduled to be relocated upstairs. If attic stairs already exist, they can be enlarged and share part of the same stairwell.
Stairs should be aligned in the same direction as ceiling joists.
Chimneys and other outlets. Existing chimneys, flues, or vents must extend higher than the new second-floor roof if they terminate within 10 feet of it. Flues and plumbing vents can be extended quite easily, but chimneys require more elaborate masonry or metal extensions, including stays or other stabilizing devices.
Setback limits. Setback requirements for your area may have changed since your home was built, requiring new construction to be set farther back from property lines. An upstairs addition aligned over your present side walls may actually encroach on present setback limits and require a variance or an upstairs addition narrower than the present house. If you obtain a variance it may include provisions for making up stairs walls fire resistant, such as in stalling wallboard under siding or installing sprinklers over windows.
Decks. Decks located over down stairs rooms create potential leakage problems because the roof can't be sloped much and the deck restricts access to the roof for maintenance or reroofing. Recently developed products, such as elastomeric membranes, are more reliable than traditional roll or built-up roofing and should be considered in designing decks. Some are durable enough for the deck surface itself, eliminating the need for a wood deck surface. Consult a local roofing or sealant company familiar with such products.
Siding material. It is generally best to match upstairs addition siding material to the existing house. Different materials may create a stuck-on look, although you can minimize this problem. First, choose material that contrasts in texture (rough board siding with stucco; smooth plywood with wood shingles; and so on). Then mix the two sidings so that one is dominant, the other an accent. E.g., you might extend some existing downstairs siding material to upstairs windowsills or extend new upstairs siding down to lower windows. Or consider using existing siding material only on the front of the addition and less expensive material on the rest. Consult with an architect or designer.
Reinforcement needs. An addition over existing living space may re quire reinforcement of the foundation and first-floor walls. Unless the design has been engineered for minimum impact to the existing structure, even modest changes have a way of expanding as new problems are uncovered during the work, or as the work itself inspires you to update every thing in its path. It may actually be wiser to gut one or two rooms at the very beginning.