Here's an article you might like to share with your builder, including some construction detail drawings. Additional resources for him or her are outlined in the yellow box near the end. I hope your construction project goes smoothly. http://www.jlconline.com/aging-in-place/building-zero-step-entries.aspx

In drememon link above, that article suggests a slight slope away from the front door so water drains away from the front door. But, the slope need not be acute. For example, the floor in porches/decks are slightly sloped downward away from house but can't be noticed by sight

Agreed, Curt. The slope needs to be increased though, from a typical porch slope, because of the lack of threshold in a zero step entry. More like a commercial building access, which, as you suggested in reference to a typical entry or porch, is sloped, but not noticeable. A big porch, to mitigate the slope, would be helpful, and a good overhang to help keep the rain out. A large entry with a gentle, virtually unnoticeable slope, and a wide sheltering roof, would say welcome home.

There are many images of this type of entry, get your builder to start researching or else get a new builder. This will always have to have some kind of threshold which if kept very low will accomadate a wheelchair no problem. If the entry way is not covered it will also have a very low slope away fom the house, not enough to impede walkers or wheel chairs but enought o keep water flowing away from the house. Don't forget when deigning this home that all the spaces are wheelchair friendly, that is showers, kitchen with nice wide doorways and space to turn a wheelchair around and also cabinets made for accessability, there is a lot to figure out so make sure you have someone to help with all of these decisions., ideally a designer with experience in accessible design.Just a couple of ideas.