As a Self-Employed mortgage applicant, there is more of a burden on you to prove the stability of your income, than for employed people. Mortgage Lenders use your income from the past two to three latest years to establish an average to try and mitigate the risk, which is why this can be difficult when you only have one year of accounts available.
In these circumstances, those lenders willing to consider your shorter trading history may ask for additional proof that your income is stable in the long-term, such as projected income for future years or your plan for business growth.
The documentation that you will need to provide in order to prove your income will vary depending on the type of Self-Employed business you carry out.

Limited Company Director

As a business owner you will need to show complete and certified accounts, SA302 forms and possibly business bank statements to prove your income. Your salary and dividends are usually used to calculate the mortgage, however, some niche lenders may be willing to consider your net profits also.

Sole Trader/Freelancer

As a Sole Trader or Freelance worker your personal income will be used to calculate your mortgage and both SA302 forms and finalised accounts will be needed to prove this income.

Partnership

As a part owner of a business, you must own at least 25% in order for the income derived from this to be considered in support of your mortgage application. Lenders will then use your share of the net profits to calculate your mortgage and they will require an SA302 form from the latest year as proof.