How is an empty room non economic? Well because it doesn't do anything.

On the other hand, a home office or child's bedroom does feed into the whole social engine where people labor to create value and prosperity that we call the economy.

Yes, the value of selling is not enough for these people to do it. That is the argument for taxation: the value of not selling needs to be more negative to spur them to action.

Another solution would be to just build more bedrooms (in general, often inside houses), but the same oldsters tend to be opposed to that too.

You are correct on the technicality, but I think OP's point is that, in combination with NIMBYism heavily restricting supply, this is a form of consumption that imposes an externality on others.