Thank you for your question.
You can claim residential finance costs on your mortgage interest payments.
This is now basically a 'tax reducer' this means you do not include as expenses,for example if your overall tax bill is say £1000, and your finance costs are £250.
We follow this example calculation £250 x 20%= £50.
£1000- £50 = Overall tax bill £950.
I hope this makes more sense.
You include under residential finance costs on the online return or SA105 box 44 on a paper return.
I have included SA105 notes to explain.
The system will work this out for you as long as you include the figure box.