Disposable income is net income minus fixed obligations with more than 18 months left (other loans, alimony), and minus rent if you will not live in the home you are buying. The payment includes earlier loans secured on the same property, from any lender. For a grace loan the bank uses the payment expected after the grace period, and for a bullet loan the monthly interest.
Income from a first-degree relative can help: if the relative guarantees the loan, passes the bank's affordability check and pays at least 20% of the monthly payment, the bank may count half of their disposable income. A spouse who lives with you and meets the same conditions can be counted in full.
The pitfall is forgetting that your other loans lower your income in this formula, and that a short term raises the payment. Many banks prefer to stay near one third of income, well under the legal ceiling.
Example
Net income is ₪20,000 and you pay ₪2,000 a month on a car loan with more than 18 months left, so disposable income is ₪18,000. A mortgage payment of ₪7,500 is 41.7%, above 40%: the bank will likely refuse or charge more. ₪7,200 (40%) is the most that avoids the extra capital charge and ₪9,000 (50%) is the absolute ceiling.