Most BTL mistakes happen because a buyer falls in love with a property before running the numbers, or runs the numbers selectively to support a decision they've already made emotionally. This workflow forces the numbers first. Follow these six steps in sequence, using the linked calculator at each stage, and you'll reach an honest answer about whether a deal stacks up — before you've spent a penny on surveys or solicitors.
Start with the achievable rent — not your hoped-for figure, but what a local letting agent confirms is realistic for the specific property. Divide annual rent by purchase price for gross yield, then subtract realistic costs (maintenance, insurance, management, void allowance) for net yield. Anything below 5% net yield needs a strong capital growth case to justify the investment; anything above 6% net yield in a stable rental market is generally a sound starting point.
BTL purchases attract the 5% additional dwelling SDLT surcharge on top of standard rates. On a £250,000 purchase this is £12,500 — a cost that's easy to underestimate if you're mentally anchored on residential SDLT rates from a previous home purchase. Add legal fees (£1,000–£1,800), survey costs (£400–£1,000), and any immediate repair budget to get your true all-in acquisition cost.
Check the monthly cost at your expected rate and LTV — but also check whether the rental income clears the lender's Interest Coverage Ratio test (typically 125–145% at a stressed rate around 5.5–6.5%, not your actual product rate). A deal that looks fine on the actual mortgage rate can still fail the lender's stress test and be declined.
This is the step most new investors skip — and the one that matters most. Net cashflow after mortgage, maintenance reserve (around 1% of property value annually), insurance, management fee (if using an agent), and a realistic void allowance (2–4 weeks per year is a reasonable starting assumption in most markets) is the number that actually determines whether the deal generates real income or just paper yield.
Combine everything into a single deal analysis — total acquisition cost, annual cashflow, and a projection of total return (income plus capital growth) over a 5 and 10-year hold. This is where you compare this specific deal against alternative uses of the same capital, including simply not buying.
Before committing, check what happens to your cashflow if mortgage rates rise 2 percentage points from today's level. A deal that's comfortably cashflow positive at 4.5% but turns negative at 6.5% carries meaningfully more risk than one that stays positive across both scenarios — even if the headline yield looks identical today.
Worked example — a £210,000 Sheffield terrace
Here's the workflow applied to a realistic 2026 scenario: a 3-bed terrace in Sheffield, asking £210,000, with a local agent confirming £1,050/month achievable rent.
This deal passes every stage: positive net yield, passes the lender's stress test, modest but genuine positive cashflow, and remains marginally positive even under a 2-point rate stress. It's not a spectacular deal, but it's a sound one — exactly the kind of unremarkable, numbers-led decision that builds a resilient portfolio over time.
The go / no-go decision framework
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