Around 83% of all landlords in the PRS (Private Rental Sector)
To elaborate, any UK private taxpayer who derives income from a property that is not their main place of residence and makes loan interest payments (mortgage, bridging loan, home improvement loan etc.) for that property.
This includes:
UK resident individual that lets residential properties in the UK or overseas
Non-UK resident individual that lets residential properties in the UK
Individual who let such properties in partnership
Trustee or beneficiary of trusts liable for Income Tax on the property profits
For more details see this guidance from gov.uk
HMRC – Who is affected?