Just because income is paid via contract does not necessarily preclude it's use in qualifying for a mortgage. The best way to present this type of "non-traditional" income is to overly document the file and make sure all other parts of the file are extremely strong.

The income should be allowed if you have a track record of working on contracts. An underwriter will generally ask for a  proven two year history of verified declared income on your federal tax returns. Another option is if you can prove that it is typical in your field or line of work to be paid on contract. If you can substantiate either of these two positions then this income should be acceptable and therefore could be used to qualify for primary residence mortgage financing.

Certainly all other criteria including sufficient assets, downpayment, collateral and credit must meet the lenders requirements for the type of loan you are seeking.