Space for children, along with their friends, would be an obvious start for consideration.
These bedrooms, closets and bathrooms should be contemplated, designed and articulated with respect to boy/girl sexes not only as youngsters, but teenagers as well (i.e. two young boys sharing a bedroom may be ok initially, but a separate additional bedroom is likely to be needed as each grows up).
Also, a trend that has been developing, is for that same teenager who graduated college out of town to now find their way back into the home, as well as older in-laws who need some special care, seeking refuge in this home. Thus strategic planning in terms of the architectural design layout/location within the home, possibly with a separate outside entrance, insulated walls, handicap bathroom specifications, etc., should be evaluated.
Another trend, but less prevalent, is to utilize space designed as a junior suite, for rental purposes. In this case, the above accommodations along with other considerations such as an additional third car garage (granted that the property can accommodate this), a small independent stack washer/dryer area within the Suite, utility sink/microwave/under counter refrigerator, service area, etc., should be planned for. Depending on site conditions, if a detached two car garage is existing, adding on to this Garage to accommodate an independent Junior Suite space, may be a viable consideration.
Logistics of the family who chooses to live in their home during a two story addition, as well as their personal schedules, must also be considered.
For instance, we are typically on the job at 7:00 a.m. and ready to perform work. On the other hand, the parents and kids are still getting ready for their day, so a request may be given for us to start an hour or more later, thus losing time from the typical day. Sure, we can work later, but with regards to the other Trades that also need to integrate first thing in the morning (i.e. material deliveries), this inherently creates some ‘stand around time’ inefficiencies.
On the other hand, a one story addition can be ‘sectioned off’ from the existing home much easier. This then allows the family to go about their schedule, with much more privacy and our construction crew can go about their schedule with ‘business as usual’.
Another consideration with one or two story, is sound. Because the one story addition is sectioned off, sounds from saws, hammers, people and material being moved around will typically be muffled within the one story addition. With a two story addition, these sounds will likely transmit much more readily, throughout the home.