You definitely want a separate alt doing the building. As soon as you start to build something, I guarantee, someone will come over and try to RP. If you are the owner, they'll also chat you up with questions or what have you. Of course, normaly you want both these things if you're not building.
Some people solve this by going 'busy' but I've heard people complain the owner should have more time for them.. It's a dilemma. Sometimes I IM people and apologize and say, I'm sorry, I just can't RP because I need to get something done. Some people just are really put off by this, though.