This is a tough question because the features and benefits of these types of projects vary greatly not to mention the fact that different areas of the country have different costs of living/taxes etc. A major cost driver is the fact that the living space is stand alone requiring all its own systems i.e. plumbing, HVAC, kitchen and bathroom. With that said, in Princeton NJ, the costs would range between $175,000 to $275,000. I hope that is helpful.