This is an intriguing renovation / addition project featured on ArchDaily — a 1960's home in Greifensee, Switzerland is part of a neighborhood with strict building regulations BUT underground buildings "don't count" as utilization of space...
Architecture firm EM2N used the existing basement and large property footprint to build a 3 room addition and patio below grade (including a slide and outdoor shower!). This greatly increased the size of the house, but did not effect the scale of the neighborhood or take away from the original neighborhood plan. Cool!
(Images: Hannes Henz)