This tool is deliberately narrower than our No Mortgage HMO Calculator: rather than annual yield and return on capital, it focuses purely on month-by-month operating cash flow and how large a contingency reserve you genuinely need, the practical, ongoing question once a mortgage-free property is already let.
This is general educational information, not financial advice specific to your property.
How this differs from an annual yield calculation
An annual yield figure smooths a whole year into a single percentage, which is genuinely useful for comparing investments but hides the month-to-month reality of running a rental property: a void month and an unexpected repair rarely announce themselves in advance, and cash flow can go negative in a specific month even when the annual picture looks perfectly healthy. This tool spreads void months evenly across the year to give an honest average monthly cash flow figure, and separately calculates a contingency reserve target, since relying on "the annual average will sort itself out" is precisely how a mortgage-free landlord gets caught short in a specific bad month.
Sizing your contingency reserve
| Property situation | Suggested reserve |
|---|---|
| Newer property, recently serviced systems | 3 months of running costs |
| Older property, or single ageing boiler/roof | 5–6 months of running costs |
| HMO with multiple bathrooms and higher wear | 6 months of running costs, given more points of potential failure |
A common mistake is sizing a contingency reserve against monthly rent rather than monthly running costs. The reserve exists specifically to cover costs continuing while income temporarily stops (a void) or spikes (a repair), so it should be calculated against the cost side of the ledger, not the income side.
Frequently asked questions
How big should my contingency reserve be for a mortgage-free rental property?
A commonly used starting point is three to six months of total operating costs (not rental income) held in an accessible reserve, specifically to cover a void period, an unexpected repair, or both occurring together. Older properties or those with a single boiler or roof nearing replacement age generally warrant a reserve toward the higher end of that range.
Does this calculator include capital expenditure like a new roof or boiler?
Not as a monthly running cost; those are irregular, large one-off items better planned through a separate capital expenditure reserve rather than smoothed into monthly cash flow. This calculator's contingency planning section is specifically designed to help size that separate reserve.
Why does this differ from the No Mortgage HMO Calculator?
The No Mortgage HMO Calculator focuses on annual yield and return on capital invested in a shared-housing conversion. This tool focuses specifically on month-by-month operating cash flow and reserve planning for an already-let mortgage-free rental property, HMO or single let, which is a genuinely different planning question.
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About the author
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