4. Rental Income & Cashflow Analysis
Cashflow analysis is the foundation of every sustainable buy-to-let decision, because it focuses on the one thing that keeps an investment alive: its ability to pay for itself, month after month, in the real world.
4.1 Estimating realistic rent
Rent should be estimated from comparable rented listings (not asking prices), the property's actual condition and specification, and local demand. Use Rightmove's "Let agreed" filter for the same property type within a short radius, because that reflects what tenants actually paid — and it's the same evidence a lender's surveyor will use. Overestimating rent by even £50–£100 a month is one of the most common causes of a fragile deal.
4.2 Adjusting for void periods
Voids are inevitable over a long holding period. Best practice is to assume at least one month vacant per year (roughly an 8% haircut on gross rent) and to hold cash to cover it. A property modelled at 100% occupancy will always look better on a spreadsheet than it performs in reality.
4.3 Gross yield vs net cashflow
These two numbers are constantly confused, and the confusion is expensive.
Gross Yield = Annual Rent ÷ Property Price
Gross yield ignores financing costs, operating expenses and risk. It's a fast way to compare two areas — nothing more. Net cashflow is what actually determines whether you can hold the property. A high gross yield with negative net cashflow is a trap, not a deal.
4.4 Worked example — a full annual cashflow
Here is the same £250,000 property used throughout this course, let at £1,300/month, bought at 75% LTV (£187,500 loan). Watch what happens to a deal that looks fine on gross yield.
Illustrative assumptions, correct as of July 2026. Mortgage rate, management fee and allowances vary — model your own figures.
| Line | Annual | Basis |
|---|---|---|
| Gross rent | £15,600 | £1,300 × 12 |
| Less: voids (1 month) | −£1,300 | ~8% |
| Effective rental income | £14,300 | |
| Less: letting agent management | −£2,246 | 12% + VAT = 14.4% of gross rent |
| Less: maintenance & repairs allowance | −£1,500 | ~10% of rent |
| Less: landlord insurance | −£300 | |
| Less: compliance (gas safety, sundries) | −£200 | |
| Net operating income (before finance) | £10,054 | |
| Less: mortgage interest | −£10,313 | £187,500 × 5.5% |
| Net cashflow (pre-tax) | ≈ −£259 | per year |
The gross yield here is 6.24% — which sounds healthy. But once voids, real costs and financing go in, the deal is marginally cashflow-negative before tax has even been considered. And for a higher-rate taxpayer, Section 24 (Module 6) can push the after-tax position further into the red, because tax is charged on a figure that doesn't fully deduct the mortgage interest.
This is the central lesson of the whole course in one table: headline yield is not profit. The same deal turns positive with a larger deposit, a lower rate, or higher rent — but you only discover that by modelling it properly before you offer.
4.5 Stress-testing the deal
A single cashflow figure at today's rate tells you almost nothing about resilience. The real question is: what happens when the mortgage rate rises at renewal? Holding net operating income constant at £10,054:
| Mortgage rate | Annual interest (£187,500 loan) | Net cashflow (pre-tax) |
|---|---|---|
| 5.5% | £10,313 | −£259 |
| 6.5% | £12,188 | −£2,134 |
| 7.5% | £14,063 | −£4,009 |
Every 1% rise in the rate costs this property roughly £1,875 a year (1% of the £187,500 loan). A deal that only works at today's low rate is structurally fragile. If it can't survive a modest rate rise, that's not a reason to hope — it's a reason to renegotiate the price, put down more deposit, or walk away.
Module summary: Base rent on real "let agreed" comparables and always model a void. Gross yield compares; net cashflow decides. A 6.2% gross-yield deal can still be cashflow-negative once real costs and finance are included — and it must be stress-tested against higher rates, because every 1% rise costs ~£1,875/year on a £187.5k loan.
Educational information only. This does not constitute financial, tax or investment advice. All figures are illustrative and rates, fees and allowances vary — model your own numbers and verify with a qualified professional before acting.