Work out what kind of lifestyle you want to have.
Do you want nights out in the pub? A cinema trip? New books, or video games? A holiday? How often do you want a takeaway? Concerts for bands you like? Weekend events? Days out? Equipment for hobbies and other related costs?
When you have a solid idea of the sorts of things you want/need for a happy life, work out how often you want to do those things, and price it all up. This will give you a figure to work with.
If that disposable income figure leaves you with enough to buy a house, then awesome! If not, figure out what you are willing to drop in order to get the house.
Backwards advice! Needs first, then wants.
Work out how much you need for a house first - that’s a need. Whether it’s rent, mortgage, saving for a deposit. And essential bills (energy, water, broadband, council tax).
Groceries and getting to work are needs - transport costs. MOT, parking, petrol, insurance, car tax, bus/train ticket etc. put enough aside for car maintenance each year.
House maintenance is a need - insurance, getting the gutters cleaned etc.
Childcare is a need to enable you to work.
Then you’ll have your disposable income - work out how much you’ve got left for the rest like savings, pension (first), then activities, eating out, clubs for the children, takeaways, holidays, birthday and Christmas gifts etc. This pot for me has fluctuated from maybe £200 a month to £1000 a month at different points through my adult life. The major bits impacting the fluctuation have been salary, nursery costs, car ownership and mortgage/rent costs particularly.