Plumbing for a sink can always be added after the fact. It will just cost a bit more. It's not a plumbing issue, since the washing machine also requires a waterline hook up. It could be an odd condo association rule. You should check that out before hiring a plumber.

Condo buildings usually have pretty strict rules, and making changes requires going through the board. If your builder is saying no, chances are it's not him being lazy it's because the condo board said no. If it's a new condo building and there's no board yet, you can always add it later.

Any additions usually have to be approved by the association managing the units.