It depends how you work.
Limited company: While many lenders use salary and dividends to determine affordability, some specialist lenders will calculate yearly income based on the day rate.
Umbrella worker: Lenders often assess affordability using payslips from the umbrella company, evaluating earnings in a more traditional PAYE way, but some may still use a formula based on the daily rate.
Sole trader: Lenders typically require at least two to three years of tax returns or certified accounts for freelancers and the self-employed.