Custom homes use a lot of standardized parts. A 2x4 is the same in Width and Depth in all the US. One of the most common lengths to buy them in is 92 5/8 - which is the perfect size when you have a 2x4 on the bottom of a wall and 2 on the top for the standard sized wall boards to fit, thus saving a lot of labor cutting each board to length. If you want a taller wall, you can also buy 104 5/8s lengths, and there is wallboard that is a bit wider so it fits exactly. If you want a wall height other than those two you will pay extra because there is a lot more labor.

Most custom houses should be looked at as custom kits. The standard gives a lot of room for the types of customization people actually want to do, while not a lot of room for the types people do anyway. Round towers look nice, but in practice they waste a lot of space compared to rectangular because nothing fits either inside or outside. (Castles went to round towers when cannons got strong enough to to knock down flat walls, for a while round was enough better in defense as to be worth the downsides)