So I just did a calculation via turn2us to assess the real financial plight someone might be in as their child reaches adulthood.
Used the example of 2 adults on 20 hrs pw on minimum wage total household income £22,000. No disability in household. Rent £1000 a month.
scenario A: 2 children at home
UC - £324.65pw = £16,881.80pa
CB - £39.90pw = £2074.80pa
scenario B: 1 child at home
UC - £252.44pw = £13,126.88pa
CB - £24.00pw = £1248.00pa
Difference £4581.72
Most people it wouldn’t be as much as this because they’d be working full time and have a bit higher than minimum wage.
But say it is about £5,000 a year difference.
For one of our DC alone we get through £3800 annually, plus childcare £1200, dental £200, £1500 activities. That is before you get to increased utility and food costs, need for a bigger house, increased transport costs, holiday costs etc
If DC reached 18 and they took on responsibility for all clothing, activities, socialising, holidays, obviously no childcare, toiletries, haircuts, transport etc…. Even after losing £5,000 we would be quids in as would most people with this burden lifted off. If they cover some food and their own lunch/food costs while out even better…
But to then say you need to charge them rent because of your dropped income… emmm nope… at least be honest and say it’s because you want to teach them a lesson about life and you want more money without working for it. Or some sort of recompense for expenses thus far.
There is an easy solution to OP’s problem - put the 20yo in full time education, sign them up for anything and your income will return, but then so will all the other costs which generally greatly outweigh that lost in benefits.
Get them to apply for UC, but at least make it fair on them and get them to move out - you don’t get money for rent if you are paying to a relative.