Buy-to-Let Mortgages: Tax Rules After Section 24 — The Section 24 reforms (phased in 2017–2020) changed UK buy-to-let economics dramatically. Many landlords now pay more tax than they realized. Here’s the 2026 reality. What Section 24 Changed Pre-2017: Mortgage interest was a deductible business expense. Landlords paid tax only on net rental profit. Post-2020 (current): …