We use a software product which is normally excellent for producing our personal tax returns and have been noticing inconsistent results from it when processing returns for buy to let landlords with mortgage interest.
The problem seems to be mostly where there are brought forward or carried forward amounts for disallowed interest (finance costs). One couple with almost identical income figures and a shared property did not get the basic rate tax deduction for 2018 because of the adjusted total income restriction but when we set up the 2019 returns for them and rolled forward the data one partner had the 2018 disallowed figure brought forward and the other didn't. Another client received the full basic rate tax deduction for 2018 but for 2019 his total income is below the personal allowance so he receives no basic rate tax deduction. His property business profits are less than the finance costs so the tax deduction would have been restricted anyway. In these circumstances I would expect the whole of the 2019 disallowed interest to be carried forward to 2020 but the software is only carrying forward the full amount less the taxable profit so ignoring the fact that two restrictions applied.
I would be interested to hear whether other practitioners are having problems either with their interpretation of what can be carried forward to future years or in the treatment of residential finance costs by their software products.
I did raise a query with our software supplier but all they could say was that they had to follow the HMRC taxonomy.