You will need foundations, but not for the reasons you think.
Assuming your planet has no atmosphere or tectonics (because it's a diamond planet and there's nothing but carbon), then the regolith (or soil) would be very much similar to lunar regolith. Countless years of bombardment by impacts and cosmic particles break down the large crystalline diamond to dust. This is not solid rock any more, but a powdery mess (even more so because it's diamond and extremely destructive to anything that touches it). You will probably need to hold the diamond powder together with some cement so you can build foundations into to, without even reaching the massive diamond basement.
A "diamond planet" is a misnomer. This does not mean that the planet is made of 100% diamond. It means that it has quite a lot of diamond compared to, let's say, earth. Also, diamond requires pressure. I would expect the crust of the planet to be dominated by graphite, not diamond. Any diamond that remains will just become graphite: The impacts on the surface on the planet (see part 1 above) will probably rapidly degrade the diamond to graphite, which is extremely soft, and will probably need to be cemented somehow so you can build foundations through it.