How to Analyse a Buy-to-Let Investment Properly
Most buy-to-let analysis stops at gross yield. Here is the full framework professional landlords actually use — with a live calculator to run your own numbers.
Ventura Research · 9 min read · Updated 15/07/2026
Key Takeaways
- Gross yield alone tells you almost nothing about whether a property will be profitable to hold.
- Stress-testing the mortgage against a higher interest rate is not optional — most lenders require it, and so should you.
- Break-even rent is a more useful number than headline cash flow when comparing two properties.
Start With Net Yield, Not Gross
Gross yield ignores service charge, ground rent, insurance, maintenance and management costs — all of which are real and recurring. Net yield, which deducts these before dividing by purchase price, is the number that actually reflects underlying profitability.
Stress-Test the Finance
Most buy-to-let lenders underwrite affordability at a stressed interest rate — typically the pay rate plus 2%. Running this same stress test yourself, using your net operating income against the stressed mortgage cost, tells you how much headroom the deal has before it turns cash-flow negative.
Calculate Break-Even Rent
Break-even rent — the monthly rent at which the property stops generating positive cash flow — is a more robust comparison tool than raw monthly cash flow, because it directly shows how much market rent could fall before the investment becomes a liability rather than an asset.
Try It Yourself
Buy-to-Let Calculator
Model yield, cash flow and returns on a single buy-to-let property.
Introduction
Who this is for: First-time and experienced landlords assessing a single rental property purchase.
Why it matters: Purchase price and headline rent tell you very little on their own — financing cost, void periods and running costs determine whether a property is actually profitable.
Typical scenario: An investor is comparing a £220,000 two-bed flat against a £180,000 terraced house, both let at similar rents, and needs a like-for-like return comparison.
- Using gross yield alone to compare properties, ignoring costs entirely.
- Forgetting to stress-test the mortgage against a higher interest rate.
- Excluding void periods and management fees from cash flow projections.
Inputs
Purchase
Finance
years
Rental
Running Costs
Analysis
A net yield of 4.5% is within, or slightly below, the typical UK regional average of 4-6% for comparable assets.
The property is projected to generate positive monthly cash flow of £61 after all costs and mortgage interest.
Rent would need to fall to approximately £1,019 per month before the deal breaks even on cash flow.
This analysis provides context, not financial advice. Consult a qualified adviser before making investment decisions.
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Sensitivity
Adjust the variables below to stress-test this scenario. Results update instantly.
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Glossary
- Gross Yield
- Annual rent divided by purchase price, before any costs are deducted.
- Net Yield
- Annual rent minus running costs, divided by purchase price.
- DSCR
- Debt Service Coverage Ratio — net operating income divided by mortgage interest. Lenders use this to assess affordability.
- Void Period
- Time the property sits empty between tenancies, during which no rent is collected.
Frequently Asked Questions
What deposit do I need for a buy-to-let mortgage?
Most lenders require a minimum 25% deposit for buy-to-let, though some products allow 20% at a higher interest rate.
Is buy-to-let mortgage interest tax-deductible?
Since April 2020, individual landlords in the UK receive a tax credit at the basic rate (20%) on mortgage interest rather than deducting it from taxable income. This calculator shows pre-tax cash flow — consult an accountant for your specific position.
What void allowance should I use?
A conservative allowance is 4-8% of the year, reflecting typical re-letting periods between tenancies.
Live Results
Gross Yield
6.00%
Net Yield
4.46%
Monthly Cash Flow
£61
Return on Cash Invested
1.20%
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View latest reports →Common Mistakes
- Comparing two properties purely on advertised rent without normalising for service charge and ground rent differences.
- Ignoring void periods entirely, or using an unrealistically low void assumption.
- Forgetting that mortgage interest tax treatment for individual landlords changed in April 2020 — pre-tax cash flow is not the same as your actual net return.
Professional Insight
The single most common mistake we see from newer investors is anchoring on the purchase price discount rather than the ongoing yield. A property bought "below market value" that then underperforms on rent and running costs for the next decade is a worse outcome than a fairly priced property with strong, durable income.
Frequently Asked Questions
What void allowance should I assume?
A conservative starting point is 4-8% of the year, reflecting typical re-letting periods between tenancies — adjust up for higher-turnover property types like HMOs or short-term lets.
Is a 25% deposit always required?
It is the most common minimum for buy-to-let mortgages, though some lenders offer products from 20% at a higher interest rate. Larger deposits generally unlock better rates.
Related Tools
Further Reading
Sources
- UK Finance mortgage lending data
- Ventura Research
Reviewed by Ventura Investment Committee · v1.4 · First published 22/03/2026
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