First let me lay your fears to rest, you will not have to open the wall. Typically the drain opening for a bathroom lavatory sink is 8" to 10" out from the wall and you want to move your sink 8" horizontal to the right or left side of its present location. That would make the diagonal measurement from the wall opening to the proposed drain position approx. 11-5/16" to 12-1/8". You could cut the existing trap adapter off the pipe extending from the wall, then install a sanitary 1/8 bend (DWV grade 45 deg elbow) and a new trap adapter. You could then install your P-trap on the sink and run it diagonal into the new trap adapter.

randy427, we had half of our house (the half with the 2 bathrooms) replumbed at the same time we had this remodel done. So it's an old house, but with new plumbing. Somebody on the Bathroom forum suggested I post on the plumbing forums of Terry Love, which I did, and there seems to be agreement that this is a venting issue. This was a gut remodel down to the studs--sure hope this is something that can be fixed without tearing out a wall! Thanks for your input.

Btw, I searched for relevant codes and found the following: R304.1 Minimum area. Every dwelling unit shall have at least one habitable room that shall have not less than 120 square feet (11 m2) of gross floor area. R304.2 Other rooms. Other habitable rooms shall have a floor area of not less than 70 square feet (6.5 m2). Exception: Kitchens. It also states: In typical houses, all living, sleeping, eating or cooking areas qualify as habitable spaces. Bathrooms, toilet rooms, closets, halls, storage or utility spaces aren’t defined as habitable, for any dwelling. But anyway you slice it, even with the remodeled floorplan the upstairs certainly meets the minimum requirements. The master bedroom is a little over 120 sq ft, and the second bedroom is over 70 sq ft (approx 90 sq ft). Even if the master bedroom didn't quite meet 120 sq ft, other habital areas such as the downstairs living room is well over 120 sq ft, so it'd just need to exceed 70 sq ft, which it certainly does. The only codes I could find regarding bathrooms were ceiling height (which wouldn't be an issue) and minimum distances between toilet, sink, shower and tub: A minimum space of 21 inches must be planned in front of lavatory, toilet, bidet, and tub. A minimum space of 24 inches must be planned in front of a shower entry. The minimum distance from the centerline of the lavatory to a wall is 15 inches. The minimum distance between a wall and the edge of a freestanding or wall-hung lavatory is 4 inches. The minimum interior shower size is 30x30 inches or 900 square inches, in which a disc 30 inches in diameter must fit. A minimum distance of 15 inches is required from the centerline of a toilet and/or bidet to any bath fixture, wall, or other obstacle. I don't see a problem with meeting any of these (depending on size of tub or shower, toilet, sink, etc.), which is why the plumbing issue seems to be the sticking point to me.

I am a designer coming up against this situation all the time...the client wants a beautiful, modern, functional vanity (storage + drawers) and the plumber comes in and destroys the vanity in order to fit in a classic P-trap with vent. No one is wrong exactly...it's just that the plumbing has not caught up with so many of our modern/sleek design solutions. Home owners and designers see vanities as storage solutions; plumbers see them as ways to hide plumbing. these are not often compatible. I am interested in teaming up with a smart forward thinking plumber to engineer some better solutions. Maybe the next time your client shows you their sleek and beautiful vanity you don't have to say "that won't work Mame"