Just as in your spouse visa entry clearance application, you will need to meet a financial requirement.
You will need to show that you or your partner has an annual income of at least £18,600 plus additional funds if you have non-British children. Meeting the financial requirement with children is covered in this post. You can rely on your own or your spouse’s income in order to meet the financial requirement, or you can combine your earnings.
The financial requirement can be met in a number of ways including through employment, self-employment, property rental income, pension income, savings etc. In certain circumstances, you may be able to rely on a combination of these. Applications where the income is derived from employment (or self-employment as a Director) from a specified limited company, or from self-employment can be slightly more complex as further documentation is required.
If you rely on cash savings alone, you will need to show certain level of savings, plus additional sums if you have children. This post covers using cash savings in order to satisfy the spouse visa extension financial requirement.