Both sources of your income would be considered for your spouse's visa. You should do a Google search for the guidance under Annex 1.7 FM, which provides all of the information for meeting the financial requirement and what needs to be submitted with the application.
Since you're employed earning £14000, you would need to show that income through official documentation as follows, according to the guidance in Annex 1.7 FM. This is the section for employment which has been ongoing for at least 6 months in the UK. If you've been employed for less than 6 months there is another section on that.
5.5.2 In respect of salaried employment, all of the following must be submitted:
• P60 (if this has been issued) and wage slips for the 6-month period prior to the
application, or as appropriate, for the 12-month period prior to the application.
• Letter from the employer confirming the person’s employment and annual salary, the length of their employment (and the period over which they have been or were paid the level of salary relied upon in the application), and the type of employment (permanent, fixed-term contract or agency).
• A signed contract of employment.
• Bank statements corresponding to the same period as the wage slips, showing that
the salary has been paid into the person’s account.
I'm pretty sure that your tailoring work would be considered self-employment. It outlines in the Annex that work can be cash in hand. Here's what I found on that:
5.4.2 In respect of earnings from self-employment:
• There must be evidence of ongoing self-employment at the date of application.
• In demonstrating the required level of income to meet the financial requirement, earnings from self-employment can be combined with income from another source(s), including salaried employment, for the relevant financial year(s).
• The applicant’s partner (or the applicant where they are in the UK) must be registered as self-employed in the UK.
• Employment can be cash-in-hand if the correct tax is paid.
• Where the self-employed person is a sole trader or is in a partnership or franchise agreement, the income will be the gross taxable profits from their share of the business. Allowances or deductable expenses which are not taxed will not be counted towards income.
• Where the self-employed person has set up their own registered company and is listed as a director of that company, the income that will be assessed will be any salary drawn from the post-tax profits of the company.
Now, I'm uncertain how it works in the UK for people earning a small, supplementary income like you do, meaning, I'm unsure if you have to register as self-employed or or if you can claim the earnings it in a different way when you file taxes. But, assuming that you would need to register as self-employed for the purposes of the visa, you would follow the guidance in the Annex for the section covering self-employment. The documentation you would need to provide would vary depending on how you're registered, i.e., sole trader, partnership, etc.
Perhaps someone else with a bit more knowledge about this sort of thing will also respond. I'm sorry I can't help further; I just don't want to give the wrong advice.
Or, you could consider meeting with an accountant who could give your professional advice on how best to prove your income.
