Family sizes are shrinking so we need more houses. Something about those maths don't add up.

Yes, if the population was to grow, but the number of people in each house was to fall, that would mean we'd need more houses.

But... If family sizes in the main are falling then that points to population decline in the longer term unless we have net immigration, which we no longer do.

There is a demand for houses when people who in the past might have shared rented accomodation decide to get their own places.

However the hundreds of thousands of empty houses and apartments around the country can meed that demand many times over.