Worth to note that there is a huge practical difference between a "custom" and a "self-build" home.
- On "self-build" you take full personal responsibility for finding land, securing planning permission, managing the budget, and hiring architects or builders.
- On "custom" you buy from a developer who already has pre-approved planning permissions, often for a huge serviced plot targeting 20-30 homes based on some copy/paste concept, managing all aspects of the construction while you can influence minor aspects of the layout, style, and interior finishes.
Custom-built homes are very common in rest of Europe. It's a common scheme to buy a plot of land, plan a low-cost project of very densely placed, similar looking houses, reusing as many elements as possible to simplify permissions, and then allowing customers to pay for additions within those fixed boundaries.
The possible individual modifications are usually minor then, everything that requires re-submission for (structural/council/...) approval is not economic.
Then there's the "I self-build, my dad is a carpenter and I just bought this toolbox" category, which is such a minefield of fatal mistakes that movies and reality shows were made around it.
I think the market for "self-build but fully outsourced to a premium developer" is the ~1-2% bracket only...