A Capital Allowance refers to sums of money a UK business can deduct from the overall corporate or income tax on its profits. These sums derive from certain purchases or investments.

Location: Self Assessment (SA100) > Create new Form > 'Self Employment' Supplementary Form > Capital Allowance tab

Refer to the link

Once the Self Employment supplementary page is open, navigate to the Capital Allowances Tab and click on 'Capital allowances Calculator.

You will then be prompted with another interface where you will need to add a new asset.

Fill in all the details as required in the below screen and save the form. On saving, the list of the assets added will be listed on the above screen, but you can edit or delete those assets should you need to.

You can also review the Assets Summary which will give you a completely breakdown of all assets added.