Sometimes the only space available when adding an addition is up! Second story additions can dramatically change the appearance and function of a house, so it's important to get it right. Here are some issues to consider when designing a second story addition:
Structural Upgrades
Since a second story addition adds additional mass on top of an existing structure, it is often necessary to structurally upgrade the existing house below. This usually means assessing the existing foundation and footing sizes to see if the foundation is capable of carrying the additional load. The existing house will often need to be securely attached to the foundation. The ability of the existing house to resist lateral movement due to earthquakes or wind should also be assessed.
The New Floor Assembly
To be less disruptive to the existing house, the existing ceiling and ceiling joists are often left in place, and a new floor structure is added on top of this. This allows for electrical lines, recessed can lights, and other elements to remain in the ceiling below the new addition. However, it also results in a very thick floor assembly. This can mean additional stairs may be necessary to reach the top floor (which take up more room) and on the exterior, special attention needs to be paid to the resulting proportions, so the house doesn't look too top heavy.
Consider Vaulted Ceilings
Many times the second story addition can use scissor trusses, which create a vaulted ceiling within the new space. If the roof is being stick-framed (rather than trusses), consider adding vaulted ceilings in master bedrooms or other spaces.
Heating and Cooling the New Space
It's not always feasible to extend existing heating and/or cooling to the upstairs, so it's often easier and better to add separate equipment for the upper level. Usually the upper level of a home is hotter than the rest of the house, so having a separate system on it's own zone will help provide for more comfort and temperature control in the new addition.
Assess where Plumbing Drains Will Go
It's important to assess how new upstairs bathrooms will connect to existing plumbing below. If there is an existing bathroom below, it may be possible to tie into existing plumbing. Otherwise, drains may need to head down through the main level to a crawl space or basement. The route of the drains, whether it be horizontal or vertical should be planned during the design phase. Many times the floor structure will need to be designed to accommodate drains (or even ductwork). This really is only feasible when done in the design stage, unless a lot of changes are made on site (which will hold up the project and add to the project cost).
Make the Stair a Focal Point
The stair is the one element that connects two levels together and it becomes an opportunity to create a dramatic space. Consider adding skylights at the top of the stairs so light floods down through to lower levels. Consider adding good lighting to make the center hall a space where art can be displayed.
Don't Necessarily Make all the Upstairs Rooms Bedrooms
A den or family hang-out space is useful to have upstairs and the space can double as a guest room if a sofa bed is used. If space is not available for an entire room, consider making a nook for a computer for kids to do homework, or parents to pay the bills.