I'm at the tail end of the strategy you're looking at - I built a house 23 years ago with a shop that was a part of the house and designed with the ability to be converted to a functional part of the house. The designer that did the plans for the house provided plans for how to convert it to a 'mother-in-law' suite, either connecting to one of the bedrooms in the main house, or with its own outside entrance. It has a bathroom, its own ac/heating system, LED can lighting, 10' ceilings and is fully insulated. The square footage is included in my property tax.
A year or so ago, I put the house up for sale. I didn't convert the shop to living space for the listing, but included the plans for converting it in the listing. I also had renderings done showing the space as a home theater and as a home gym. I thought it'd be restricting the pool of potential buyers if I converted it to any one of these configurations, so I left it as a shop, but put 2'x3' prints of the renderings/plans on poster board on the walls of the shop (the shop is dry-walled, has a seamless epoxy floor and painted the same color as the interior of the house - it is quite nice, if I do say so myself.)
The results of all this effort? Not much. It was shown by realtors an average of about twice a week, which was quite good for this market. I thought for sure there'd be someone like me looking for a shop space for a car project, a home theater fan who wanted the space for his dream project, someone who needed to have their parents live with them but wanted to keep their privacy, or an exercise nut that wanted a home gym. In all the feedback I got from my listing agent, there was very little mention of the shop or any questions about converting it to livable space. It didn't seem to matter.
I priced the house at the average cost per square foot for my area, and included the attached shop at about 50% of that value. The premium on the house price for the shop was in the range of about 7-10 % and the shop added about 15% to the overall living space of the house if converted. I had lots of showings but only a couple of offers. The offers were a couple percent off the livable square feet x going price per square foot - I couldn't 'monetize' that shop space.
I concluded that-
- Men don't buy houses, women do, and they don't give a damn about a shop, home theaters, gyms. They seem mostly concerned about the kitchen and master bath.
- You can't sell potential - it would have to be converted to true livable space before being able to extract some dollars for it.
- The minute you have it converted to some specific use, you restrict the pool of potential buyers
Bottom line? Build whatever you want and enjoy the hell out of it for as long as you're there, but don't count on getting much more back for it than you're spending to build it. Except for the master bedroom sleeping, I spend more time in my shop than any other part of the house. I've had more enjoyment with the $30K or so it cost me to build my shop than almost anything I've ever spent money on.