After much experimentation, ive managed to build underground, ive so far got 3 stories below surface level. Ive put a very stylish swimming pool at the bottom, with a loungs area in the centre. Ive managed to make the top level just pop out of the ground, so from the outside, it only looks 1 story high. I'll upload my save of it, and some screenies soon. Anyone else tried doing this yet?
First you have to decide how many levels you want below ground, and thru trial and error, lower the terrain to be exactly low enough to house the structure. I decided to make a structure withing another so that the surface layer had a patio and staircase. Its a little difficult at first, i spent sveral hours getting it right. Sadly my taste in asthetics isnt great, so it could look better, but overall im pleased with managing to build underground.
Is there a stair tool you can use? In the Sims 2 you could use it to dig down without any fuss. Just place one set of stairs at ground level, pointing down, then put another set at the bottom of that, and one more at the bottom of that. And that's exactly one floor. Delete the stairs, then level the ground off to create your space, and repeat until you have the number of floors you want.