Aside from the significant associated increase in maintenance costs on e.g. the roof that would come with such an option, I bet you the market would be fine with that. Stamp out houses that are all alike except some have less bedrooms enabled. Hell, offer the extra bedroom capacity as a rental option.
If the customer only paid for 1 bedroom, they're going to save a lot of money. It's the extra maintenance costs of that roof and the associated space taken up by the structure that would make it a harder sell, otherwise dynamically growing living space would be very interesting.
You think builders are going to create rooms people might not buy and not pass off the cost to the buyers, or development company? You have more faith in companies or maybe builders than I do.
Also, how will you keep me out of the room in my house that I didn't buy? You can't effectively.
Capitalism has done way dumber things than that, so I don't think it's impossible that company A builds 100 houses or apartments exactly the same and company B sells them as different sizes, based on how much the customer pays.
A creative tenant could certainly break through a wall that's been put up in place of a door, but that seems pretty extreme. I've seen some creative construction projects to make use of crawl spaces that weren't originally designed for people to live in though, but that's far from the norm.
If the customer only paid for 1 bedroom, they're going to save a lot of money. It's the extra maintenance costs of that roof and the associated space taken up by the structure that would make it a harder sell, otherwise dynamically growing living space would be very interesting.