One problem with designing your own house is that you don't necessarily know what it's going to be like until it's finished. The company we hired to build ours doesn't make model homes the way some housing companies do. It's one of the reasons we chose them, since models add to the price of the product. All we had were two-dimensional printouts based on computer models.
It wasn't a big problem. Our design was simple, basically a box with three rooms on the first floor and one large room on the second. But because we weren't sure what it would be like in its finished form, we left off a lot of things that most housing companies tend to include as a matter of course, such as landscaping and other exterior (gaikō) features. We wanted to see the actual building in its environment before we made those decisions. The housing company could have thrown in an approach and a parking space for a little extra, but it would have been standard issue. If we wanted something more idiosyncratic, we would either have to do it ourselves or contract out. In either case it would be more expensive than what the housing company would have offered.
When you buy a home from a developer, landscaping and exterior features are usually part of the deal. The buyer will have some leeway in terms of materials and maybe which trees are planted in the garden, but in most cases exteriors in a subdivision are uniform since the developer can buy materials in bulk and have the same landscaper design everything. In those cases, we found, exteriors cost between ¥1 million and ¥2 million, depending on the size of the plot. Ours is 22 sq. meters, and we wanted to pay up to ¥2 million.