I've found that most landlord P&L discussions start from gross rent and never get much further — which is why real buy-to-let profitability is so often miscalculated until something goes wrong.
Calculating real buy-to-let profit requires subtracting every cost from gross rent before drawing any conclusions about whether the investment works. Most landlords — especially newer ones — know their gross rent and their mortgage payment. The other costs, individually modest, add up to a figure that materially changes the profit picture. Here is the complete calculation in order.
Start With Gross Rent
Gross rent is the amount your tenant pays, before any deductions. For a property renting at £1,050 per month, gross annual rent is £12,600. This is the top of the calculation — everything else reduces it.
Use a realistic rent estimate based on current market comparables, not aspirational or historical figures. If the property has been tenanted at the same rate for three years without a review, the current market rent may be higher or lower. The calculation should reflect what you can actually achieve in today's market for a property in current condition.
Subtract Mortgage and Finance Costs
For leveraged investors, mortgage interest is almost always the single largest cost. Buy-to-let mortgages are typically interest-only — meaning monthly payments cover only the interest, with the principal repaid at sale or on remortgage.
Monthly mortgage interest = outstanding balance × annual rate / 12. On a £180,000 interest-only mortgage at 5.3%: £180,000 × 0.053 / 12 = £795/month. Annual finance cost: £9,540.
After mortgage interest, the annual rental income of £12,600 has been reduced to £3,060 — before any other costs. This is why gross yield figures can be so misleading for leveraged investors: the financing cost is the dominant variable, and at high mortgage rates it can absorb the majority of rental income entirely. Use the True Buy-to-Let ROI Calculator to model how the cash flow changes at different LTV and interest rate combinations.
Include Repairs, Insurance, and Fees
Maintenance and repairs provision: Budget 1% of property value annually as a floor. On a £240,000 property: £2,400/year. This covers routine maintenance, redecoration between tenancies, appliance replacements, and minor structural items. Major unexpected repairs — boiler replacement, roof repair, damp remediation — should be covered by a separate larger contingency reserve rather than treated as exceptional when they arise.
Landlord insurance: Buildings insurance, liability cover, and optional rent guarantee insurance. A comprehensive landlord policy typically costs £200 to £450/year depending on property value, location, and tenant type.
Letting agent fees: Full management (tenant sourcing, rent collection, maintenance coordination, legal compliance): 8% to 15% of monthly rent. On £1,050/month at 10%: £1,260/year. Tenant find only (used between managed tenancies): typically £900 to £1,500 as a one-off.
Compliance costs: Gas Safety Certificate (annual, £60 to £100), Electrical Installation Condition Report (every five years for new tenancies, £150 to £300), EPC renewal (every 10 years, £60 to £100), right to rent checks, and any licensing fees in areas with selective or additional licensing schemes (£500 to £1,500 per property where applicable).
Account for Void Periods
Void periods — time between tenancies when no rent is received but costs continue — are a certainty, not a risk to be optimised away. Even with excellent tenant retention, most properties experience at least two to four weeks of void per year across tenancy transitions, redecoration periods, and occasional extended gaps.
At £1,050/month rent, one month void per year reduces effective annual rent from £12,600 to £11,550 — a reduction of £1,050. For a property with higher tenant turnover, two to three months of void per year is realistic, reducing effective rent by £2,100 to £3,150. Budget for at least one month of void annually in any cash flow calculation.
Calculate Real Monthly and Annual Profit
Pulling the complete calculation together for the example property (£1,050/month rent, £240,000 value, £180,000 mortgage at 5.3%):
- Gross annual rent: £12,600
- Less: mortgage interest: -£9,540
- Less: maintenance provision (1%): -£2,400
- Less: landlord insurance: -£300
- Less: management fees (10%): -£1,260
- Less: void allowance (1 month): -£1,050
- Less: compliance costs: -£200
- Pre-tax annual profit: -£2,150
This property is pre-tax cash flow negative. After adding tax on rental income (minus restricted mortgage interest relief), the position worsens further for higher-rate taxpayers. The gross yield was 5.25%. The actual return on cash invested depends on capital appreciation — without it, the investment loses money annually. This is the calculation that matters.