I also wonder what will happen to all those who own overseas property or have retired in Europe, guessing they will have to come back? Border controls work both ways.
ECHR (nothing to do with the EU) gives everyone right to peaceful enjoyment of their possessions. A house is a possession. It cannot be taken from them summarily.
International law prevents one country from expelling any sector of its habitual population. To do so is deemed a 'crime against humanity'.
But forget all that and start thinking for yourself -
Do you think Spain, for instance would take all the houses off the British owners because they have left the EU and leave ghost towns up and down the Costas, just a few years after the big Spanish housing slump - of course not.
Do you think the Spanish will kick out all the British pensioners whose pensions contribute to the Spanish economy and create jobs for Spaniards in Spain - of course not.
Do you think Spain will kick out all the British who are working in Spain and whose skills contribute to the Spanish economy? Highly unlikely. I would imagine the businesses that employ them would raise merry hell with the Spanish Govt if it proposed that and they suddenly had to fire, recruit and retrain. However, if the Spanish Govt really want to do that they will be free to do so. Just as we, as a free country, making our own immigration laws that allow us to invite anyone from across the world with the skills we need to come and live in the UK, could equally say - but not if you're Spanish. Not that we'd want to, nor would we, - but we could.
If you think things through you realise that a lot of the 'they'll kick everybody out' is just more project fear scare-mongering.
We have to stop seeing ourselves as people who have things done to them and start to realise that, with restored sovereignty, who does what to whom is a negotiated settlement.