For a two-bowl vanity, it's 15" minimum between the centerlines of the two sinks, as well 15" from the centerline of each sink to the adjacent wall on either side of each sink. That's an IRC code minimum. Those specific numbers probably won't be found in the actual plumbing code, as the plumbing code is more geared towards the actual plumbing and not the layout. IRC, or the actual building code, is the code that contains minimum space requirements. As far as code hierarchy, IRC is for the most part the building code that covers the United States as a whole. Individual states then have their state codes. Most states simply embrace IRC as a whole, though some states add some additional restrictions geared towards that state. Example, a state like California may have added sections that address seismic concerns. Then there is local code. Individual towns or cities may have additional restrictions dealing with their very localized issues. Historical neighborhoods, etc. For the most part, local and states codes can't relax national (IRC) code requirements, but they can be more restrictive. Sometimes code is added for political reasons. Example, the plumbers union in Massachusetts has a very strong influence on code there. Same with electricians and the electrical code in Chicago. Restrictions are added to try to make it difficult for DIYers to do any work on their own houses.

It shouldn't be a big deal, but you would have to first remove the old vanity and then open up the wall and do some re-plumbing. You would cap off one drain and one set of hot and cold pipes in the wall. Then you would need to route the other drain and set of hot and cold pipes laterally within the stud space so they would be in the center; this might require notching out one stud, depending on where things are located. Which set of pipes you would choose to cap off might depend on where things were in the wall in relation to the studs: it's possible that one side might be easier to re-route to the center than the other, or one side might be a shorter run to the drain. It's hard to say before you open the wall, but that's the general idea. Once you re-did the plumbing and repaired the sheet rock, you would be ready to install the new vanity. Oh, and if you don't have shutoff valves under the sinks, use this as an opportunity to install a set on the pair of supply lines you will be using. Hope that helps. Feel free to re-post if any of this is vague, or once the wall is open.

Can you downsize a double vanity to single vanity? Of course you can! I assume you currently have a long vanity with a sink at each end. If you're planning to place a sink on one end and leave the other end as counter space (with space for make up and hair), it'll be easier: Your plumbing is already in place for a sink on the end. However, if you want the sink to be in the middle, you'll have to move the plumbing over a couple feet. Hard to know if it would hurt resale value, as everyone is looking for something different. My mom went to look at a condo today, and one bathroom had a plastic shower surround, and the MB had a platform tub which she did not like. She hated them both. Yeah, someone else might've considered the plastic shower surround a positive since it's so easy to clean and the platform tub might've been his or her personal preference. Within the realm of common sense, don't remodel for resale; you'd be guessing! I think people like his and her sink for their master baths and double sinks for bathrooms that are used by the whole family - one sink for the parents, one sink for the kids. Guest baths surely don't need two sinks for resale. Not me. I don't want duplicate sinks anywhere. They're unnecessary, they eat up space that could be used for storage, and they double the chance of a plumbing leak and a repair bill.

I prefer two mirrors as well because I think it gives a more balanced look and it provides defined spaces, his and hers in our case. Your statement about wanting to make it so that 3 people can use it is what has me curious though, is this a what if type scenario or is a 3rd person actually going to be in there all the time? The reason I ask is because DH and I used to make choices based on the what if's a lot. We'd spend the money or compromise on what we really wanted only to never have those "what if's" materialize. We bought a crew cab truck so if we needed to we could tote people but never once did anyone sit back there. We turned a room (that could have been my craft room) in to a guest room and in 10 years it's only been used a handful of times and an air mattress would have worked just as well. So if you want the big mirror because it will actually be used in the context you're planning for, great...then there's a need and that would be the more functional choice for your situation, but if you're just thinking well maybe 3 people might need it, then don't focus on that and do what is more practical for your every day living. KWIM? (And Ida is right, about the two lights)