In the first part of the example, these are entries on my 2023/24 test account:
Firstly, you must have your SA105 property page completed with your mortgage interest entry in the Mortgage interest expense group and ensure you have correctly entered this under the residential property entry point:
As a simple example, here is my property page, showing £9,000 income and the only expense being the mortgage interest (of which will mean there is some left over to carry forward).
Once you have completed your tax return, you should go to the Reports page and view your calculation:
Here you can see I have highlighted the maximum allowable in blue and the actual amount used in red.
To work out how much you can carry forward to your next tax return, you should take the maximum allowable and deduct the amount used.
Here you can see £12,000 is the maximum allowed and £4,430 is the amount used.
Following a simple calculation 12,000 - 4,430: £7,570 is available to carry forward.
------------------------------------------------------------------------------------
In this part of the example this is my 2024/25 test account:
How to enter the £7,570 unused mortgage interest on this tax year:
Navigate to the property page and the mortgage interest expense group, and select 'Unused finance costs brought forward' and enter the amount we calculated earlier:
You should also enter the mortgage interest for this tax year under Mortgage interest for residential properties:
Which should give you are completed property page like this:
The mortgage interest will then be offset against the taxed profits of the property income in the tax calculation:
Once again in blue you will see the total of mortgage interest available and in red the amount being used in this tax year. Again, after a simple calculation, you will see £2,281 is unused and can be included on my 2025/26 tax return as unused brought forward interest.