"Ignoramus18484" wrote in message

Thanks. What kinds of structural issues are possible? What Iam
concerned about is, foirst, if the external walls are strong enough,
and second, if the middle of the house supports are strong enough.


It could be fine as it is, you might need more support on the outside walls
to carry the weight, you might need one or more "bearing walls" added to
the inside, or you might just need a large beam added here or there.

Then the building department can be pricks and make you change the size of
an existing beam... just to be pricks.

An architect can advise you best on this stuff.


Would it be comparable with, say, the cost of building a typical 3
bedroom house on a slab?


Cost can vary a great amount depending on what the architect determines
should be done. Also upgrades to electrical, air conditioning, etc. In some
cases it is easy, in other cases a lot of work would need to be done... or
entire systems upgraded to new ones. Is your electrical at capacity? Or is
there amperage/breaker space for the addition? Again, ask the architect....