If you work from home, rent or mortgage costs can be claimed as part of HMRC's flat rate, so how much you can expense will depend on hours worked from home.
If you're using your home as your small business premises, you can use HMRC's flat rate which varies, depending on how many hours you spend working from home:
HMRCs flat rate covers any rent and utilities. It does not cover phone or internet costs - you can expense the share of those that relates to your business.
Specifically for Landlords:
Mortgage interest for any properties or rooms sublet is no longer tax deductible from rental income. Since 2020, the buy-to-let rules have fully transitioned to a tax credit system. This means you can claim 20% of mortgage interest (and other financing costs, such as arrangement fees) as a tax credit. earnr can help with this!