Section 24 is the name commonly used to mean the reduction in mortgage interest relief for buy-to-let investors.
The changes mean that mortgage interest (and other finance related costs) can no longer be deducted from income as standard expenses for landlords. Instead a separate relief is given at 20%, regardless of the tax rate you're paying.
The new rules were phased in from the 2017/2018 tax year with full effect in 2020/2021.
There are more details on the gradual implementation on our blog.
Calculating your own impact from the Section 24 tax changes is unfortunately complicated. Generally speaking you'll need to investigate the rules in detail or seek advice from a qualified accountant or tax advisor.
PaTMa Property Manager includes an estimate of the potential effects for your individual portfolio, although this does not allow for the effects from your other taxable income.
You can also use the free PaTMa buy-to-let profit calculator to calculate an estimate for an individual buy-to-let property.