Most but not always, second story additions are way more involved and costly. It all depends on your current footprint and the property it sits on. In some cases, folks have no other alternative but to go up because of property line constraints. Second stories involve a lot more engineering because of loads and the concern that your current footings/foundation supports will handle the upper story loads. Your first step would be to consider the advise from a structural engineer to see if a second story is possible with the existing, or how much would be involved with getting it structurally sound. That might make the decision for you single story add- on vs. second story add- on.

Sorry I should explain..
We currently have a 2 story colonial. I want to either add on a large single level addition or a 2 story adjacent to what we already have. Not put on a second story to our current house.

Hi Nadine, If i understand you correctly, you're looking at the possibility of extending the existing second story, or adding on below. If that's the case, then the second level is still quite a bit more involved.If you alraedy have an existing bath upstairs, that's a plus. If not, you would probably want to consider an upstairs bath which involves plumbing tie-ins. Your existing footings/ foundation supports could be okay for that unless they only went with the calcs on the existing second level loads. In any event, it would still be a good idea to get it analyzed by an licensed engineer in your area. It typically runs around $300.00 in our area for a consultation. It might differ in yours.

Build 2 level... same amount of lumber, drywall, electrical, floor covering, etc.

One foundation, one roof!

If I was bidding this job, I assure you the two level is cheaper. Tieing into the existing is usually not a big deal with the exception of matching roof covering.

"one foundation, one roof"

Assuming the existing foundation is calc'd for the new second story load.

I've yet to see a second story built on top of a pitched roof below. Seems to me there's quite an effort to remove that concerned section of roof in order to joist/sub floor/frame the second deck. A single story add on even with the new foundation is less costly in most cases that i've been involved with.

Nadine, no one will be able to give you $$$$ comparisons as there are too many variables and would need extensive details from you. Your best bet is to get estimates from a remodeling contractor/builder(s).

I understand the OP is adding onto the existing house with an addition that is entirely outside the footprint of the existing structure.

The OP has not included critical information, i.e., where (s)he is located. In parts round me, a residential structure is always built on a basement (or four-foot frostwall). In that case, the standard foundation design will support either a one or two story house/addition.

A 500 square foot addition with all 500 feet on the ground floor would cost less than a 500 square foot addition that had 250 square feet on the first floor and 250 square feet on the second floor.

However, IMO, when taking on a project like this, it is much more important to consider the spaces obtained, their uses, and their value to you now and in the future, as well as the value of the improvement to a future buyer. For example, having a first floor bedroom (and bath) might be worth the extra cost of building it all on one floor.

I guess it's still not clear. The 2 story would NOT be adding onto existing foundation. It would be a two story addition on a whole new foundation and I'm trying to figure out if it's cheaper to do one large room addition (new large foundation) or a totally new two story addition tied it into our current house (new smaller foundation). I'm located in the northeast just outside Boston. I would try to plan on just a slab foundation, we already have a basement, and there's no reason to enlarge it. The plan would be for a single floor addition: expand the half bath on the first floor to a full bath and add on a family room and small bedroom. For a 2 story addition, have the family room on the bottom floor and the bedroom above it. We could still either expand the 1/2 bath on the first floor to a full bath or add on a small full bath upstairs too. (we already have a full bath upstairs, but we'll really need an additonal shower in the future as the kids get older). It seems like there's conflicting info here....with a 2 story it would be less foundation and less roofing, but I know a two story addition probably requires a stronger foundation and the strength of the ceiling to hold up the second floor.

Outside of Boston requires a footer that is constructed 4 feet below the ground surface, so you would end up with a four foot frost wall, or for not much more money, a full basement. Such a wall would support a single story or a two story home no problem.

The bottom line on foundation construction in the northeast is that a one-story and a two story foundation are built basically the same, and the difference - if any - is negligible.

You do NOT want a concrete slab on grade. In New England, nearly ALL construction is done over a crawlspace or basement, and slab-on-grade constrution is not the standard used in this part of the country due to the weather.

You want a floor framed with 2-by wood lumber, generally 2x10s or 2x12s, depending on layout. You would need this for the floor whether you are 500 sq. ft. on one floor or 250 sq/ft each on two floors. The size and number of 2x floor framing members is about the same either way you go.

And going two stories you save half on ceiling joists 'cause the 2nd level floor joists are also the ceiling for the first level and the roof goes in half, almost.