Even with your example of a late 30s couple with 2 school age children on an average income there's huge variation and you have to look at your individual circumstances.
If you live in an expensive area and have little equity in your home, paying for wrap around care, you could already be feeling stretched, such that moving somewhere bigger or spending on lifestyle could be difficult/unwise. Or if you live in a cheaper area and/or have a low mortgage due to inheritance, big deposit, previous higher income etc etc it could be doable.
Also need to consider preferences re lifestyle. If you could only have one of the bigger house or nicer holidays what would it be? How do you spend on things like cars, bills, groceries, clothes, grooming, days out, eating out etc etc. The range of costs for these can be significant.
How secure is your income. How likely is it you coul lose your job and how quickly could you get another one? If you were on long term sick, how long would you get paid for?
Remember that you can only spend each pound once and you should aim to spend less than you earn. Also plan for ups and downs in expenditure and save for irregular costs. Pay for cars, holidays, insurance etc with savings not debt where possible.
Look at the financial flow chart as a 'to do' list for money, this guides you through the steps in the right order.
ukpersonal.finance/flowchart/