How to Project Cash Flow on a Buy-to-Let Portfolio

2026-08-11

Yield Is a Snapshot. Cash Flow Is the Film.

Ask any buy-to-let investor their yield and they'll tell you instantly. Ask them what their portfolio cash flow looks like in 2031 and you'll get a blank stare. That's a problem, because yield tells you where you are today. Cash flow projection tells you whether you'll still be profitable in five years when your fixed rate expires, rents plateau, and Section 24 has fully eaten your tax relief.

This guide covers how to build a realistic cash flow projection for a UK buy-to-let portfolio — the variables that matter, the ones people ignore, and how to model scenarios that actually prepare you for what's coming.

The Variables That Drive Buy-to-Let Cash Flow

A cash flow projection is only as good as its assumptions. Here are the inputs that matter, in order of impact:

1. Mortgage Rate and Term

This is usually the largest single expense and the most volatile variable. If you're on a 5-year fix at 4.2% expiring in 2028, what rate will you refinance at? The difference between remortgaging at 4.5% and 6% on a £150,000 mortgage is £187/month — £2,244/year per property. Across a five-property portfolio, that's £11,220 per year of cash flow variance from a single assumption.

Model at least three scenarios: optimistic (rates fall 0.5%), base case (rates hold), and stressed (rates rise 1%). If your portfolio goes negative in the stressed scenario, you have a problem that needs addressing now, not when it happens.

2. Rental Income and Growth

Rents in the UK have grown at an average of 2-4% per year over the past decade, with significant regional variation. London has been flat in real terms while northern cities have seen stronger growth. Don't assume your area matches the national average — check ONS data for your specific postcode.

Be conservative. Model rent growth at 2% per year, not the 5% some areas have seen recently. Recent growth rates reflect a supply crunch that may ease. If rents grow faster than your projection, that's a pleasant surprise. If they don't, you're prepared.

3. Void Periods

A property earning £1,000/month with one month's void per year doesn't generate £12,000. It generates £11,000. That's an 8.3% haircut. Most landlords dramatically underestimate voids, especially when projecting forward.

Budget 4-6% for well-located single lets with strong demand. Budget 8-10% for HMOs where individual rooms turn over more frequently. Budget 10-15% for properties in weaker rental markets or with unusual characteristics.

4. Maintenance and Capital Expenditure

The industry rule of thumb is 10-15% of rental income for maintenance on older properties, 5-10% on newer builds. But this average hides lumpy costs: a boiler replacement (£3,000), a roof repair (£5,000), or a full redecoration between tenants (£2,000) don't arrive on a smooth schedule.

In your projection, use a percentage of rent for routine maintenance, but also include a sinking fund allowance for major capital expenditure. Setting aside £100/month per property for future big-ticket items is more realistic than assuming maintenance stays flat forever.

5. Section 24 Tax Impact

If you hold properties personally (not in a limited company), Section 24 is likely the most important variable in your projection and the one most investors still don't fully model.

Mortgage interest is no longer deductible from rental income. Instead, you get a 20% tax credit on the interest paid. For basic-rate taxpayers, the effect is neutral. For higher-rate taxpayers, it's devastating. On a property with £6,000/year in mortgage interest, a higher-rate taxpayer loses £1,200/year compared to the old regime. Across a portfolio, this can push profitable properties into an after-tax loss.

Even worse, Section 24 can push you into a higher tax band. The untaxed rental income (before the tax credit) is added to your total income, potentially pushing you from basic rate to higher rate — or into the £100,000+ bracket where your personal allowance is tapered away.

Your cash flow projection must model this. A property that looks profitable pre-tax can be loss-making after Section 24 for a higher-rate taxpayer. If you're not modelling this, you're not projecting — you're guessing.

6. Insurance, Management, and Compliance Costs

These are smaller individually but they compound:

• Landlord insurance: £200-400/year per property, rising 3-5% annually
• Management fees (if using an agent): 8-12% of rent
• Gas safety certificate: £60-80/year
• EICR (every 5 years): £150-250, so £30-50/year amortised
• EPC renewal (every 10 years): £80-120, so £8-12/year amortised
• Selective/additional licensing (if applicable): £500-1,200 per licence period

In total, compliance and insurance costs typically run £500-800/year per property, and they only go up. Model 3-5% annual inflation on these costs.

Building the Projection

A useful cash flow projection models monthly or annual figures for each property across a 5, 10, or 25-year horizon. At minimum, you need:

• Row 1: Gross rental income (current rent × growth rate, less voids)
• Row 2: Mortgage payments (fixed for current term, then modelled at assumed refinance rate)
• Row 3: Operating costs (maintenance + insurance + management + compliance, inflated annually)
• Row 4: Net cash flow before tax (Row 1 - Row 2 - Row 3)
• Row 5: Tax liability (rental profit × your marginal rate, less Section 24 credit)
• Row 6: Net cash flow after tax (Row 4 - Row 5)

The magic is in Row 6 projected forward. A property that nets £300/month today might net £50/month in three years when your fix expires, or £500/month if you remortgage well and rents grow.

What Good Looks Like — and What Should Worry You

A healthy buy-to-let portfolio shows positive cash flow in all three scenarios (optimistic, base, stressed) across the projection period. The stressed scenario might be tight, but it shouldn't be negative.

Warning signs in a cash flow projection:

• Cash flow goes negative when your fix expires: You need to plan for this now. Can you overpay the mortgage, build a cash reserve, or sell a weaker property to deleverage?
• Section 24 pushes you into a higher band: Consider incorporating. The cost of transferring to a limited company (SDLT, CGT, refinancing) may be justified if Section 24 is costing you thousands per year.
• Maintenance costs are unrealistically low: If you've budgeted 5% on a 1960s terrace, you're kidding yourself. Increase it to 12-15% and see if the numbers still work.
• No sinking fund for capital expenditure: Boilers, roofs, and kitchens don't care about your cash flow. They break when they break.

The Decision the Projection Should Drive

A cash flow projection isn't just a planning exercise. It should drive concrete decisions:

• Should I remortgage early to lock in a better rate? The projection shows the break-even.
• Should I sell a property to reduce portfolio leverage? The projection shows which property is weakest.
• Should I incorporate? The projection shows the cumulative Section 24 cost over 10 years versus the one-time transfer cost.
• Can I afford another property? The projection shows whether the portfolio cash flow supports additional borrowing.

Every serious portfolio landlord should have a projection they update at least annually. The market changes. Rates change. Regulations change. Your model needs to change with them.

Build the model. Stress-test it. Let the numbers guide the strategy.