When a friend of mine bought his first buy-to-let property he told me he was getting an 8% yield. He had divided the annual rent by the purchase price and was pleased with the result. Six months later he was considerably less pleased. The boiler had failed in November, the tenants left a month early, and the letting agent had charged fees on the void period he had not read in the small print. His actual return for year one was negative. He had not done the maths wrong — he had done the wrong maths. Rental property investment is genuinely worthwhile for many people, but only if the calculation starts with cash flow, not the headline yield figure.
What Yield Tells You and What It Hides
Gross yield is annual rent divided by purchase price, expressed as a percentage. It is the figure quoted in property listings and used for quick comparisons between locations. A property bought for £180,000 generating £900 per month in rent has a gross yield of 6%. The problem is that gross yield ignores every cost that sits between the rent arriving and the money reaching your account. Net yield adjusts for those costs: letting agent fees, maintenance, landlord insurance, buildings cover, void periods, and mortgage interest if you are using financing. A 6% gross yield property can easily produce a 3–4% net yield once those deductions are applied — a figure that changes the investment case significantly.
Mortgage Costs Have Changed the Numbers Since 2017
Buy-to-let mortgages work differently from residential mortgages in a way that now matters a great deal for profitability. Before 2017, landlords could deduct mortgage interest as a business expense when calculating taxable profit. That relief was phased out and replaced by a basic rate tax credit, meaning higher-rate taxpayers now pay tax on rental income before accounting for their mortgage interest. The practical effect is that a landlord with a large mortgage and a higher-rate tax bill can end up paying income tax on a property that is breaking even — or even making a nominal loss — in cash flow terms. Running the numbers through a rental yield calculator that accounts for financing and tax is now essential, not optional.
Void Periods Are Not an Unlucky Exception — Budget for Them
Every landlord experiences void periods: weeks or months when the property sits empty between tenancies. In a well-managed property in a strong rental market, void periods of two to four weeks per year are typical. In a slower market, or after a difficult tenancy that requires refurbishment, voids can run considerably longer. A property generating £1,000 per month sounds reassuring until a six-week void wipes out £1,500 in one go. Most experienced landlords budget for one month of void per year as a baseline, and hold a separate cash reserve for exactly this purpose.
Short-Term Lets and Airbnb: Higher Revenue, Higher Complexity
Short-term rental platforms can generate significantly more revenue than a long-term tenancy in the same property — sometimes two or three times as much per month in high-demand areas. But the comparison is not as straightforward as it looks. Short-term lets require furnishing the property to a high standard, managing bookings, cleaning between stays, handling guest queries, and dealing with the seasonal nature of demand. Occupancy rates below 60–70% can make the maths worse than a long-term tenancy, not better. Platform fees also run at 15–20% for fully managed services. A direct comparison between long-term and short-term returns for your specific property and location is more useful than the headline nightly rate.
Hidden Costs Landlords Forget Until They Arrive
The expenses that catch new landlords off guard tend to share one characteristic: they are irregular and therefore easy to exclude from the mental model of monthly cash flow. A new boiler is £2,000–£4,000. A roof repair can be £3,000–£8,000. Redecorating between tenancies typically costs £800–£2,000 depending on the property. If you are using a letting agent, fees for tenant finding, tenancy renewal, and property management can total 15–20% of annual rent. EPC improvements — now required to meet minimum energy efficiency standards — can involve significant upfront expenditure for older properties. Building all of these into an annual maintenance reserve of 1–1.5% of property value is a practical starting point.
Cash Flow First, Capital Growth Second
Many landlords justify thin or negative monthly cash flow on the basis that the property will appreciate in value. Property has historically appreciated over the long term in the UK, but appreciation is neither guaranteed nor predictable on any particular timeline. A property that loses £200 per month in cash flow while theoretically gaining in value requires you to fund that monthly loss from other income. If the market plateaus for five years — as it has in many regions at various points — the strategy becomes very uncomfortable very quickly. The safest approach is to require a property to be genuinely cash-flow positive after all realistic costs before acquisition, and treat any capital growth as a bonus rather than a rescue mechanism.
Before You Buy: The Numbers to Run
Before committing to a purchase, calculate net yield after all costs and financing; monthly cash flow after mortgage payments and expected expenses; the time to break even on purchase costs including stamp duty, legal fees, and any refurbishment; and how the numbers change if the interest rate rises by two percentage points. Stamp duty on buy-to-let properties includes a 3% surcharge on top of the standard residential rates, which significantly affects the entry cost on properties in higher price bands. Getting to a decision without modelling these scenarios is how investors end up with properties they cannot sell without a loss and cannot hold without monthly pain.
What to do next
Use the ideas above as a starting point — then connect them to your own numbers and related guides on Calc It Anything.
- Read the mortgage, home buying and property costs guide for the wider cluster.
- Compare with Is This Rental Property Actually Worth Buying?.
- Compare with Rental Yield vs ROI and Why Property Numbers Get Misunderstood Constantly.
- Run the relevant calculator on this site with your own inputs before making a decision.
Related reading
- mortgage, home buying and property costs guide
- Is This Rental Property Actually Worth Buying?
- Rental Yield vs ROI and Why Property Numbers Get Misunderstood Constantly
- House Poor Explained: When a Home Starts Controlling Your Entire Financial Life
- Complete Rental Property Investment Guide
For official UK context, see GOV.UK buying and selling a home.
Frequently asked questions
Should I compare gross yield or net cash flow first?
Gross yield is a quick filter; net cash flow after mortgage, voids, maintenance, and tax is what determines whether you can hold the property comfortably. Stress-test both before you offer.
How much of my income should housing take?
A common planning band is 25–35% of net household income, but high-cost areas and variable-rate mortgages may need a lower target. Model your own numbers rather than copying a rule of thumb.
Is overpaying a mortgage always better than investing?
Not always. Compare your mortgage rate after any tax relief with expected long-run investment returns, your emergency buffer, and how long you plan to stay in the property. The right answer depends on your numbers and risk tolerance.