Get a good architect and a home inspector. I would get the suggestion on the inspector from an architect so they could discuss some of waht you have in mind. It will all take longer than you thought and cost more than you thought. But it will be an adventure. The slimness is certainly reflected in the price, but you are going to make it yours, I woould plan to make sure there is a lot of light to give it more expansivness

People with experience have told me that if it's a narrow townhouse, it should be ornate on the exterior for the best resale value. If it's a plain Federal on the outside, it will be less valuable.

I have two townhouses on my website, one a beautiful rehab, the other needs some serious work but has a tenant in place. You might want to take a look at their prices just for comparison purposes.

I'm also told that rehabbing shells is very expensive ($700,000+), and there could be tax implications if the unit becomes a multi-family when it is currently being taxed as a single family.

The upshot is, I think, to do your homework as much as you possibly can.

As I recall, the poet Edna St. Vincent Malay lived in a very, very narrow townhouse in Greenwich Village (somebody correct me if I misremember this!) If the location is appealing and there is good light, I don't quite believe that narrow-ness alone will be a major turn-off for potential tenants. After all, a lot of apartments have bedrooms or living rooms that narrow.