Buy-to-let running costs checklist (UK): what to budget beyond the mortgage

Why running costs matter more than “headline yield”

Many buy-to-let decisions start with yield, but the day-to-day reality is driven by cashflow and running costs.

Running costs matter because:

• they reduce net yield
• they reduce cashflow available to cover mortgage costs
• they can be irregular (“lumpy”) rather than smooth monthly expenses

This guide is a practical checklist you can use when evaluating a property or reviewing an existing one.

Key terms (quick definitions)

• Net yield
• Void period
• ICR

The core checklist (what to include)

1) Maintenance and repairs

Most properties need ongoing maintenance. Even if you have a “good year”, maintenance tends to arrive eventually.

Practical budgeting:

• include an annual maintenance allowance
• include a “shock” repair buffer (boiler, roof, damp)

2) Letting / management fees

If you use an agent, fees can be a recurring percentage of rent. Even self-managing landlords often have some costs (advertising, tenancy setup, contractor admin).

3) Insurance

Landlord insurance (and any additional cover you choose) is part of the ongoing cost base.

4) Service charges and ground rent (if leasehold)

Leasehold flats can have service charges that materially reduce net yield and cashflow. Treat service charge as a first-class input.

5) Compliance and safety costs

Depending on property and situation, there can be safety checks and compliance work. Treat these as ongoing costs, not one-offs.

6) Voids (vacancy)

Voids are one of the biggest hidden risks.

Even “one month void per year” is ~8.3% of annual rent. If your plan has no buffer, voids can push you negative quickly.

7) Mortgage costs (stress tested)

Even if the deal rate looks fine today, a landlord plan should stress test:

• higher interest rates
• different lender stress assumptions

Use the Buy-to-let mortgage calculator and Mortgage interest to explore rate sensitivity.

A practical cost model you can reuse (four buckets)

Bucket A — fixed-ish recurring

• insurance
• agent/management fees
• service charges (often recurring, but can change)

Bucket B — variable recurring

• repairs and maintenance
• compliance/certification work

Bucket C — vacancy risk

• void allowance (percentage or a “months empty” assumption)

Bucket D — finance

• mortgage payments (cashflow view)
• interest-only stressed interest (lender-style view)

If you model these four buckets, you’ll usually have a more robust picture than a single “net yield” number.

Worked example: how costs change the story

Assume:

• Property value: £250,000
• Rent: £1,250/month → £15,000/year

Gross yield: 6.0%.

Now add annual running costs:

• maintenance allowance: £1,500
• insurance: £300
• agent fees: £1,200
• void allowance: £1,250 (1 month)

Total annual costs: £4,250.

Net yield (excluding mortgage): (£15,000 − £4,250) ÷ £250,000 × 100 ≈ 4.3%.

If you add mortgage payments, net cashflow could drop further. This is why “headline yield” is not enough.

Stress testing (what to run before you rely on the numbers)

Use three scenarios:

1. Baseline: your best estimate.
2. Rate stress: +1% to +2% interest rate.
3. Void + cost stress: 1–2 months void + higher maintenance year.

If the plan breaks under scenario 3, you may still proceed, but treat it as higher risk and ensure your buffer is real.

What to do next

• Use Rental yield to model gross vs net yield with your cost assumptions.
• Use Buy-to-let mortgage to test rent-based borrowing assumptions.
• Read: Rental yield explained.

Expanded checklist (more detail, fewer surprises)

If you want a more complete checklist, consider these additional categories.

Letting setup and tenancy admin

Depending on how you let the property, you might pay for:

• advertising
• referencing
• tenancy agreement setup
• inventories / check-in / check-out processes

Even if you self-manage, these costs often show up as time + occasional paid services.

Ongoing property upkeep (the “lumpy cost” reality)

Some costs don’t happen monthly, but they are predictable over the long run:

• redecorating between tenancies
• appliance replacement
• flooring wear
• roof and gutter work

The safest budgeting approach is to assume a “good year” does not mean costs are gone — it means you are between costs.

Leasehold-specific risk (flats)

If the property is a leasehold flat, cashflow can be dominated by:

• service charges
• major works contributions
• management changes

Practical steps:

• request the last service charge budget and recent history
• ask whether major works are planned
• include a buffer for service charge increases

Vacancy/void planning

Voids hurt twice:

• rent stops
• costs continue

If your mortgage payment and running costs are high relative to rent, even short voids can flip the deal negative.

Practical approach:

• model 1 month void per year as a baseline
• model 2 months void per year as a stress scenario

Rate risk (mortgage stress test)

Rate rises are one of the fastest ways to break a buy-to-let cashflow plan.

Even if your initial deal is fixed, you should model:

• +1%
• +2%

Then ask: “would I still be okay if this lasted 12 months?”

A “minimum viable landlord spreadsheet” (copy/paste structure)

Use this template for each property:

• Value / purchase cost base: £_____
• Monthly rent: £_____
• Annual rent: £_____
• Annual costs (non-mortgage): £_____
  • maintenance allowance
  • insurance
  • letting/management
  • service charge / ground rent (if any)
  • compliance allowance
• Void allowance: £_____
• Mortgage payment assumption (or interest-only stressed interest): £_____
• Net cashflow per month: £_____

Then add a second column:

• Rate +2% scenario
• 2 months void scenario

If your cashflow is only positive in the best-case column, the deal is fragile.

Another worked example: cashflow under stress

Assume:

• Rent: £1,100/month
• Mortgage payment today: £850/month
• Other costs averaged: £200/month

Baseline cashflow ≈ £1,100 − £850 − £200 = £50/month.

Now stress test:

• mortgage rises to £980/month at a higher rate
• 1 month void per year (average −£91.67/month)

Stressed cashflow ≈ £1,100 − £980 − £200 − £91.67 = −£171.67/month.

This is why a “small positive” baseline is often not enough. You need a buffer.

A practical decision rule (not advice, just a sanity check)

If your plan depends on:

• no voids
• no repairs
• no rate rises

…then it’s not a plan; it’s a hope.

A more resilient approach is to target deals that stay broadly okay under:

• +2% rate
• at least 1 month void
• a higher maintenance year

Summary

Running costs are where buy-to-let plans succeed or fail. If you budget for maintenance, voids, insurance, service charges (where relevant), and rate risk — and you still have buffer — you’re building a landlord plan that can survive real life.

Compliance and safety costs (budgeting mindset)

Compliance requirements and safety checks vary by property and situation. The key budgeting mindset is:

• assume there will be ongoing compliance-related costs
• avoid treating them as “one and done” forever

Even if you don’t list every certificate and rule in a spreadsheet, you can include an annual allowance so you’re not surprised.

Property type matters (how costs differ)

Houses vs flats

Flats often have leasehold costs (service charges) that can dominate the running-cost picture. Houses often have lower management-structure costs but may have higher direct maintenance exposure depending on condition.

Newer vs older properties

Older properties can have more maintenance uncertainty. Newer properties can still have costs, but the “shock repair” risk may be different.

Single let vs multi-let (where applicable)

Multi-let or higher-turnover strategies can increase:

• tenant turnover admin
• void frequency
• wear and tear

The higher the turnover, the more important it is to include realistic “lumpy cost” allowances.

A “questions to ask before you buy” checklist

Use these questions to turn uncertainty into known numbers:

• What is the realistic rent (not the best-case rent)?
• What does the property cost to insure?
• If leasehold, what are the current service charges and are major works planned?
• What is the minimum cash buffer you want to hold after completion?
• Does the deal still work with 1–2 months void?
• Does the deal still work at +2% interest?

If you can answer those, your plan is far less likely to be fragile.

Portfolio mindset (even if you only buy one)

Even with one property, it helps to think like a portfolio manager:

• “What happens if two bad things happen in the same year?”
• “Do I have cash buffer for a major repair and a void?”

Landlord plans often fail because multiple small stresses happen together.

Worked comparison: two properties with similar yield, different risk

Property A:

• Rent: £1,200/month
• Service charge: £0 (house)
• Maintenance allowance: £1,800/year

Property B:

• Rent: £1,200/month
• Service charge: £2,400/year (flat)
• Maintenance allowance: £1,200/year

On headline rent, they look identical. But Property B starts with a built-in cost drag and potentially more “outside your control” risk through service charges.

This doesn’t automatically make B worse — but it means you should demand more buffer in the numbers to compensate.

Maintenance planning: a simple way to avoid panic years

Instead of “hoping repairs are low”, treat maintenance like sinking-fund thinking:

• set an annual maintenance allowance
• keep a separate buffer for major repairs
• expect irregular years

If you build a buffer in good years, you can handle bad years without taking on expensive debt.

A monthly cashflow checklist (what to watch once you own the property)

Once the property is running, keep an eye on:

• rent received vs expected
• void days per year
• maintenance spend vs allowance
• service charge changes (if any)
• mortgage rate changes and upcoming deal end dates

The earlier you spot drift, the easier it is to fix.

Final takeaway

If you can explain your buy-to-let cashflow under conservative assumptions in one sentence — “it stays okay with +2% rates and one void month” — you are in a much stronger position than someone relying on perfect conditions.

Copy/paste: annual cost checklist (quick version)

Use this list to make sure you haven’t missed categories:

• maintenance allowance
• “shock repair” buffer
• insurance
• letting/management fees
• service charge and ground rent (if leasehold)
• compliance allowance
• void allowance
• mortgage cost stress test

If you can fill in a number for each line (even a rough one), your model is usually more realistic than most headline-yield comparisons.

Worked example: turning a property into an annual budget

Assume:

• Rent: £1,250/month → £15,000/year
• Insurance: £300/year
• Maintenance allowance: £1,800/year
• Management fees: £1,200/year
• Void allowance: £1,250/year (1 month)
• Service charge: £0/year (house)

Total annual non-mortgage costs = £5,? (here: £300 + £1,800 + £1,200 + £1,250 = £4,550).

Then:

• Net rent before mortgage = £15,000 − £4,550 = £10,450/year

If your mortgage payments are £900/month, annual mortgage is £10,800.

Annual cashflow ≈ £10,450 − £10,800 = −£350/year.

In this example, the deal “almost works” — but it’s negative once you include realistic costs and a void allowance. A small improvement (lower purchase price, higher rent, lower borrowing, lower fees) could make it work. The key is that you only see the truth when you model the full cost picture.

One-sentence summary

If you budget buy-to-let running costs realistically (maintenance + voids + fees + rate stress) and you still have buffer, your plan is far more likely to survive real life.

That’s the point of this checklist: not to make deals look good, but to avoid surprises that turn a “good on paper” investment into a stressful one.

If you’re comparing multiple properties, use the same cost model and the same stress tests for each. Consistency is what makes your comparisons meaningful, and it helps you spot the deals that only work because the assumptions were quietly changed.

If a deal only works when you assume zero voids, low repairs, and today’s lowest rates forever, treat that as a red flag. A resilient landlord plan is built on conservative assumptions and a real cash buffer.

That buffer is what turns a spreadsheet into a strategy you can actually live with.

It also helps you sleep at night when the boiler fails.