I have found another option that falls somewhere in-between building your own house and choosing a pre-fabricated one. On the one hand you can design the house any way you like, and on the other hand you can still build it quickly as if it was pre-made and only had to be assembled. As the matter of fact it looks a lot like LEGO.
This option is offered by ZEGO, they offer hollow foam blocks that you use to build the “shell” of your house and then fill those with concrete. Their website has a video that explains everything in a really fun and easy manner (click here to watch). It says that one level of a house can be built in one week. They even lay out a costs comparison table, have a look. According to that table, ZEGO wall is the cheapest (100$ for square meter) compared to brick veneer (111$), double brick (169$) or concrete block (142$).
Besides the advantage of quick construction they also mention that the house becomes more energy efficient (because it is better insulated) and consequently home owners saves on heating and cooling costs (up to 80%!). Plumbing and electrical works are made easier because of the form blocks (no need to drill in concrete) and the house is 4 time stronger than traditional house.
All in all, I’d say that ZEGO definitely worth a look.