How can I connect a rooftop with ground?

Solution 1:

If memory serves, this is a limitation of the game in that even if you raise the ground to the roof of the "below ground" section of the house. that below ground section is still the first floor, on top of it is the second floor, and the raised section of ground is the first floor again. The two can not meet by means other than a set of stairs.

However, one thing you should be able to do is lower the raised ground slightly and put a foundation wall on top of it. Everything on top of the foundation is also considered the second floor, and if the top of the foundation and your 'underground' garage are at the same height, you should be able to connect them.