How would this dictatorship in which people have to give up the homes they've paid for actually work?
Will there be separate ghettos for single young people, the coupled-up but childless/childfree, small families, large families, the working with adult children who might need to come home for a while, the early retired who do a few hours a week and a shift or two at the foodbank, and pensioners, so that nobody has a house that someone else has deemed to be too big for them? Will Granny have to move again when Grandpa dies, even though they both left their lifetime home three years previously?
Who is going to build all of these developments, and what happens to the existing housing - will it only be 2 bed starter homes that survive the cull, as they are suitable for both the young and the old? Who gets to decide how big is 'too big'? Are we looking at an age of New Communism, where the State decides who can live where and controls personal freedoms? The trouble with that, is that being able to buy yourself out of staying put by paying punitive taxes is incompatible with Communism (in its pure and non-corrupt form, at least - perfectly fine in a State-controlled dictatorship, obviously).
What about second homes/Air B&Bs/holiday rentals? If people can't have a bedroom more than they 'need', can anyone have more than one house? And is that per couple or per person?
Who is going to live in houses in the middle of nowhere, if families don't want them because there are no facilities, but they are 'too big' for couples? Will we all be concentrated on housing estates in towns and cities?
Will there be transport between the ghettoes, so that grandparents can babysit and pick up grandchildren from schools and nurseries?
Will houses lose value when the owners are being forced to sell, and the buyers know that the vendors have to be out by a certain date? (NB that will apply to all the groups, so when a child is getting to 16 or so, the parents will have to be thinking about selling up when they reach adulthood).
What if the unthinkable happens and a child dies? Will the parents have to sell up immediately, or will there be a grace period?
When older parents have to sell to young families who have new mortgages and childcare expenses, so have to reduce the selling prices accordingly, how will this impact on the inheritance they can leave to their children? This might suit the New Communist regime, but it will require a huge shift in expectations for those expecting to inherit at some point.
What is the incentive going to be for anyone to buy a house if they know it will be taken from them down the line and is no longer going to be security in old age, (or even until when the first child leaves home)?
Will there be as much work for builders or decorators etc, when people heading for Eviction Day don't see the point in getting their paintwork done or the roof fixed for the sake of a couple of years? Will it just be those in the more settled stage of this Brave New World who can look ahead more than five years or so, with everyone else knowing they will soon have to move on to free up their accommodation for those in the relevant lifestage demographic.
What if when the time comes, you can't afford to move? Will it be ok to have a third child if you live in a 2 bed, when there are expectant parents looking for one, or is the compulsory sales in one direction only?
Will there be extra police on the Aged Persons' Ghettoes, or will they be left to fend for themselves against criminals who know where they live?
Or is the compulsory nature of this scheme only going to apply to the elderly who can't afford the punitive taxes, and every other age and financial group gets to choose where to live, have security of tenure in their own house and get to decide whether they need a spare room or not?