I've been involved in selling finance for many years, and when correctly completed all finance application forms (including mortgages and bank loans) will include a detailed list of OUTGOINGS, thereby arriving at the applicant's DISPOSABLE income.

This is the only way in which a lender can ascertain whether the prospective borrower is able to COMFORTABLY meet the repayments.

It was of course the widespread ignoring of these basic requirements which ultimately led to the collapse of the banking industry, especially in the USA.

Now though at first it may appear more complicated for the govt. to base their income requirements on disposable income, in fact they actually have more detailed (and confidential) information to hand than does the average mortgage lender or bank...especially in the case of those submitting a visa application !

As usual, our silly ex-prep school MPs have little knowledge of commerce and their methods of sorting the wheat from the chaff (when applied correctly ).