Experience Adding On to a Home?

Still looking for the perfect house here

Right now we are considering one that will eventually be a little small for us if we have another child. Also, I want a large bathroom, and because I don't want to take too much space out of the home (It is only 70 square meters) I would want to immediately add on a separate bathroom and perhaps build a basic storage shed on the property.

Do you have experience adding onto a home as opposed to just renovating the interior?

Any clue how much a nice, large bathroom would cost (Underfloor heating, Jacuzzi tub, moderately priced tiles) as an addition, not just a renovation? I know nothing of foundations and building techniques and if it is even possible to add a room to the exterior of a home without MAJOR construction?

Also, is there special permitting we would have to get to add on a bathroom and a room to the house?

Any experience with this would be great, because it will let us know if we should keep looking for a larger house or expand our search to include slightly smaller homes that we can expand in the next 5-10 years.