Best idea (which i think is the first reply) is to get a design from an existing house and modify it.
not just because modifying is easier, but normally when you go through the process of dealing with a builder they'll have a price for a house plan, but if you work off changes to that plan then you can just work off how much more/less expensive those changes will make to the house.
It also means that for large proportions of the house they'll know exactly what they're doing and that really helps when it comes to when they build it which helps for both them doing it right the first time, and how quickly the house comes along.
Last time my parents used OwnIt homes
. These were the best builders out of four block up builds they've done. So i'd have to promote them both for the build time (on schedule/ahead of schedule) and their post-build support (ie issues afterwards like the exhaust hood on the stove and a few other bits) was quite excellent. The post build inspection before the keys are handed over is huge too - they covered the house with red stickers - giving it measles with every little bump and scratch or scuff that had been picked up and they fixed all of those before they handed the keys over.
Anyway have fun.