Buying a rental property is one of the largest financial decisions most people make, and it is often driven by emotion: a nice building, a persuasive agent, a friend's success story. A calm analysis of the numbers takes an hour and can save years of regret. This guide sets out a simple method you can repeat for any property.

Step 1: Estimate the rent honestly

Look at what similar units in the same area actually rent for, not what the seller hopes. Check listings, speak to agents and ask neighbouring landlords. Then allow for vacancy. Few properties are let for all twelve months, so a vacancy allowance of one month a year, or more in a weak market, is prudent. Your realistic income is the rent less the empty months.

Step 2: List every cost of buying

The purchase price is only part of the cost. Add legal fees, stamp duty, valuation and survey fees, agent commissions, transfer costs and any repairs needed before letting. The costs of buying property checklist lists them, and stamp duty on property transfers explains the duty. Land transfer process in Kenya, step by step shows the sequence and the due diligence in land due diligence checklist protects you from a defective title.

Step 3: List the running costs

Every year the property costs money even when it is let: land rates, insurance, repairs and maintenance, agent fees, service charges for an apartment and the cost of finding tenants. See land rates and county property charges and service charges and sinking funds. A common rule of thumb is to allow a fifth to a quarter of the rent for costs, but use the real figures for your property.

Step 4: Calculate the yield

Gross yield is the yearly rent divided by the purchase price. Net yield uses the rent after costs and is the number that matters. A property with a gross yield of 9 percent may have a net yield of 6 percent once costs, vacancy and tax are counted. Rental yield explained walks through both. The rental property investment analyser works all of this out from your inputs, and lets you compare properties side by side.

Step 5: Add the tax

Rental income is taxable. Decide whether you will pay the monthly rental income tax or file an annual return and deduct expenses, since the result differs. Use the rental income tax calculator to compare, and read MRI or annual rental tax. If a tenant is a company, it may withhold tax from the rent; see withholding tax on rent: what landlords and tenants should know. When you eventually sell, capital gains tax applies, and you can estimate it with the capital gains tax calculator.

Step 6: Test the financing

If you borrow, work out the monthly repayment and compare it with the rent. A property that earns more than it costs, after loan payments, is cash-flow positive; one that does not has to be funded from your pocket every month. Check the interest rate, the fees and what happens if rates rise. Mortgage interest relief applies only to a home you live in, as explained in mortgage interest relief for homeowners.

Step 7: Consider growth and risk

Rent and values can grow, but they can also fall. Ask what is happening in the area: new roads, shops and schools raise demand; oversupply lowers it. Consider how easy it would be to sell, and whether the property depends on a single employer or institution for tenants. Off-plan purchases carry extra risk; read buying off-plan property: risks and checks.

Compare with other options

Ask whether the same money would do better elsewhere, for example in a savings product or in paying down debt. Property is illiquid and demanding to manage, so it should offer a meaningful premium over safer choices. Should you buy a home or keep renting? covers the related decision for your own home.

Make a decision rule

Before you start looking, set your minimum net yield, your maximum borrowing and your maximum purchase price, and stick to them. Write down what would make you walk away. A clear rule keeps emotion out of the decision, and a simple spreadsheet keeps the arithmetic honest. If the numbers do not work, the right answer is often to wait for a better property.