Also trading up comes with stamp duty, which is a damper on moving house unless you absolutely have to. Not only is your new house bigger and more expensive then anything you've ever bought before with that extra bedroom, plus the large expenses and hassle of moving in general, it's likely another £10k or more in stamp duty unless your new 3-bed is somewhere pretty cheap in the country. You don't get the first time buyer discount either, as you already have a house.

It's basically a tax that punishes labour mobility and not oversizing property.

Of course, you have to pay to heat and maintain that bigger house and pay higher council tax on it, so your saved stamp duty will probably be used after a few years, so in an ultra-rational way, it comes out in the wash, but it certainly doesn't feel like it at the time.

House ownership is higher among boomers, who are not going to have children anyway. Maybe for some people concern for future prices is a good reason, but certainly it is not for everyone. If there’s spare capacity enough for 20% more people and population growth forecast by 2050 is just one third of that, you cannot really talk about undersupply, when there’s enough accommodation to cover even future demand.

Also consider that many 3rd bedrooms in the UK are tiny. Like, "don't fit single bed frames and allow the door to open properly" tiny.

Rooms used for another purpose are not called bedrooms. See yourself: https://www.ons.gov.uk/peoplepopulationandcommunity/housing/...

UK census differentiates between rooms and bedrooms, so indeed an unused bedroom is likely a dead space.

Lol bedroom police. Yes, because that's exactly what we were discussing.

Or, you could, you know, make it more affordable to build smaller homes with fewer bedrooms so people actually have options when they become empty nesters.

My point is that the relatively static proportion of people to dwellings in those countries over that period suggests that a supply shortage is not sufficient to explain the accommodation crisis.

What I am suggesting is that incentives to invest in property have led to an increase in the number of properties that are not being used as a primary residence (i.e. are underutilised) and that that is the most significant factor causing the shortage of available homes.