For married couples, it is mandatory for husband and wife to sign as co-borrowers. For single applicants, it is optional. However, having a co-borrower with regular income increases the chance of getting approval, especially for applicants with low income or below average credit. A co-owner of the property has to be a co-borrower, but a co-borrower needs not be a co-owner. A co-borrower can be a spouse, siblings, parents, and children. A friend, an organization or a company is ineligible to be a co-borrower.