on the flat
1. On a level plane, i.e., without slopes or hills. I'd actually recommend building the house into the side of the hill—if you build it on the flat at the bottom, you run the risk of flooding during heavy rainfall. They've designed the marathon so that the majority of it is run on the flat.
2. In horseracing, taking place in an open course without jumps. The jockey is much better on the flat than he is in races with jumps.