Not necessarily, depending on where it is & the rules ... usually you'd build something onto both boundaries, and houses on those blocks are often arranged front-to-back (ie. you don't have rooms next to each ... you have them one-behind-the-other). If you think about it that way, you can have a 1.5m wide hallway & the rooms can still be 6m wide, or even a 1m hallway with 3m rooms on either side ... and the living area & kitchen are usually combined so that can be as big as the kitchen/living/dining in The Edge at Homeworld 5.
Of course, fitting a standard project-home design onto such a block is an entirely different matter ...