It depends on circumstances which are variable. DS and DIL - late 20s, he's an academic, she's in marketing earn £35k and £40k respectively. They chose to live a couple of tube stops to London. They rent a one bed flat in a mixed HA development on a commercial rent. They clear about £3900 pcm between them:
Rent £1800
Utilities £250
C Tax £250
Phones £80
Netflix/Broadband: £50
Insurances £50
Fares £200
Food £350
Car (tax, insurance, maintenance, permit) £200 (no allowance for replacement)
So their outgoings, before they pass go are about £3,200 That leaves them with about £700 for savings, spends, nights out, holidays, gifts, etc. I appreciate the car is a non essential but whilst they live only just over 10 miles from us, on public transport it takes 90 minutes, by car 30 (at weekends).
I only know their budget because dh and I went through it with them.
In London, if there were a property to finance in the commercial sector, with at least two bedrooms, costs would be higher depending on location - the further out, the higher the fares. At £85k probably no WTC or Child benefit. Plus with dc, more heads to feed, childcare costs (can be £100 per day in London) and other child related expenses.
It would be different if a family were in social housing and eligible for universal credit, working tax credit, child benefit, etc.
I don't think DS and DIL could start a family, living in Central London based on their income of £75k or even £85k if they had no other help. I think it would be equally difficult if they moved to zone 4/5 also but they would get more space.
It isn't about the overall earnings but about what is left after essential expenditure.