My parents recently (September) completed a weekend house at our family farm. And while some stuff definately added to the overall cost (all the doorways are 4' wide, the hallways are 6' wide- so it DID increase the overall square footage) other things (like a walk-in shower without a lip on it and a slightly dipped floor), making the counters a bit lower than standard wouldn't be much more expensive than any other custom option- and when you're having your own house built, it's silly to think you'll want everything off the shelf anyway.

The people who really need working on are the large homebuilders. Custom homes aren't as much a problem as the less expensive housing like subdivision and tract homes, where designers seem to choose the least expensive solution for any given option- 24" doors for closets, the tiniest, least expensive showers and tubs available, grab bars which are yes, sunk into studs- but the studs are 2x4 instead of really being well-anchored (thye're something that may be supporting the weight of an adult human weighing more than a hundred pounds- not all disabled folks are skinny little old women!)

I'm hoping to take some photos of the new house soon, if you'd like them. It's a beautiful home and the accessibility is just a bonus- it adds to the airy, light feel of the house.