They are not dead spaces, and they wouldn't somehow become more useful if I knocked a wall or two through to reduce the room count or reclassified them.
I don't know anyone that has an empty room in their house, or one that no-one enters for weeks at a time, etc. Maybe the odd country estate is like that.
To a communist, the concept of having a guest room, office, storage room etc might feel like "underutilization".
If we apply that more generally, then the park outside my house is underutilized because the maximum capacity are not sunbathing in it at all times. I think that's a pretty silly use of language.
I'll never understand how someone could think that completely normal things like a house, garden, car, etc are somehow "too much". But they do. It's baffling.
I can understand, though not agree, with being angry with someone who owns say, tens of thousands of houses, millions of acres of land, and leaves it all empty.
But the idea that a house is underutilized if it doesn't have as many people as could possibly live in it? All I can say is, Hong Kong exists, Manhattan exists, feel free, I'm not in for that.
This is true. The problem tends to be that people become settled in an area, and would prefer to downsize there so they don't need to uproot themselves. This is often very difficult, as there may not be availability in that area for a suitable smaller property.
I don't have any statistics to hand but, based on experience, there are a lot of houses in the UK like this. Particularly among middle class couples in their 60s, whose children have left home. In fact, my parents have two spare bedrooms that are only used a handful of times each year.