20YO Rent??

[nightowl22]

My 20YO step son has typically lived between our house and his mums, so max 3 times a week. In the last 6 months him and his girlfriend have alternated between 3 houses, and we haven’t charged rent because of this.
They have recently asked us if they can move in with us full time while they save for a place of their own. I’d be keen to get people’s thoughts, experiences and also ideas on how much rent to charge them both, to cover food, increase in bills etc.

My parents had the rule that if you earn enough to pay tax you earn enough to pay rent.
At 30 I moved back to my parents as my job started 6 months earlier than my partners. I was there Monday -friday and at my home over the weekends. I think I paid £400 a month everything included. I was on a good salary and paying rent on another house and had already saved a deposit for a house.
With a 20yo I would have a sit down with them and go through their outgoings and work out what a sensible proportion of their salary should go into savings and what goes to rent. I would go through the general household bills with them so they can see what the rent covers and what they benefits they have by living with you