If you are going to add a second story, is your foundation rated to handle the load? That would be my first question.

My second question would be is the load bearing wall strong enough to handle the added weight?

The answer to these two questions will determine whether you go up a story or add on to the main section.

Enjoy the journey.

eal51 in western CT

Assuming your foundation and existing walls can handle the load, building up is usually cheaper than building out (no additional foundation costs for one thing). It may also be cheaper to heat a two-story building of a certain square footage than a one-story building of the same square footage, because the exterior square footage of the walls and roof, as well as the first floor area, is less for the two-story building. Less surface area means less heat loss area.

Thanks...I guess I knew it wasn't going to be easy to answer with no data. :) Good point about the heating, though.

You might also want to consider what effect a second story additon would have on your neighbors. If yours is a large lot, perhaps not much. On the other hand, if adding a second story deprives your neighbors of privacy or sunlight in their back garden....you'd probably want to give that some thought.

If you do decide to go up, in addition to the foundation and heating cost savings, you'll also reduce the amount of roof to be shingled and the amount of guttering needed.

On the other hand, you'll lose the square footage on the ground floor taken up by a stair.

Both have Pros and cons.
A friend that's a contractor told me it's about the same cost going up or out.Although no foundation going up, there is demo work, new joists etc. And as pointed out you must first know if the load can be handled.
Pros Extending up
heating maybe cheaper of course,
You don't lose any yard ( that's a big one to me)
Con's
Building up disturbs your whole house and maybe harder to live through. It also causes the need to paint most rooms and repair ceilings.
You lose ground floor space for stairs.
Extending out:
Pros No stairs.No loss of space for them.
Con's where you extend (old meets new) will need to be repaired
Loss of yard
heating
I think the choice should depend more on your house your property and what you want to get out of it. People sometimes don't consider the floor plan or flow of the house. Not all additions are created equally. Some homes may look and flow best building up, others by building on. My house would look too much like a commercial building if I go up. You must consider flow and the look of your house. Someday you'll want to sell it. If your house looks like it was added to without regard to appearance you'll have difficulty selling it. Think it through carefully. Cost of construction may not be the most important aspect. In the long run the best way your house will flow and look would be the best financial decision.

Thanks. I know I couldn't base much of my decision on cost. We don't have a large yard, so I'd love to keep what we have (it's big for this area though). Fortunately the neighborhood has been around long enough to see what looks OK from the outside and what doesn't--everything (permissible by the HOA) has been tried by someone. I'll just need interior tours.

I wouldn't mind losing ground floor space for stairs. We could just cannibalize one of the dinky bedrooms. The public rooms are spacious, thanks in part to a ground floor addition 50 years ago.

Obviously I'll need to talk to a pro, but it's good to start thinking about all the things I need to think about!