No money question carries more feeling than this one. “Rent is throwing money away” versus “a mortgage is a millstone” — both slogans, both wrong as stated, and both usually argued with numbers that compare the wrong things. The classic mistake is weighing rent against the whole mortgage payment — but part of every mortgage payment buys equity back, and renting frees a deposit to compound elsewhere. Apples, meet orchard.

The model that compares like with like rests on one idea: unrecoverable costs — money that leaves each month and never comes back, under either roof.

The standing line, meant more than ever here: this models the finances of the biggest purchase of most lives. It organises the decision; a mortgage adviser and your own judgement finish it. And it deliberately says nothing about the part spreadsheets can’t hold — security, roots, the landlord’s boiler.

Renting’s unrecoverable cost

Easy: the rent. Plus contents insurance, minus nothing — the deposit you didn’t spend stays invested, and its return offsets the rent. Monthly:

rent_cost = rent + insurance − (deposit * invest_return / 12)

That last term is the one folklore forgets: a £40,000 deposit earning a real 4% quietly hands the renter £133 a month.

Owning’s unrecoverable cost

Harder, because the headline payment is a blend. The unrecoverable parts:

own_cost = interest + maintenance + buying_costs_monthly
           + ownership_extras − expected_appreciation/12
  • Interest, not the payment — your amortisation table’s interest column, which is most of the payment early on. The capital slice is transfer to yourself, not cost.
  • Maintenance — the folklore-free estimate is ~1% of the property’s value per year. Owners fix their own boilers.
  • Buying costs, spread — stamp duty, legal, survey; £8–15k that amortises over your expected years in the house, which is why short stays favour renting almost regardless of the rest.
  • Appreciation — the offset on the owner’s side, and the model’s most dangerous input: real (after-inflation) house price growth has long-run averages near 1–3%, not the folk-memory double digits. It gets a named cell and a hard stare.
renting, per month rent £1,150 insurance − deposit return £133 net ≈ £1,040 owning, per month interest £780 maintenance £240 buying costs £95 − appreciation £120 net ≈ £995 capital repayment appears in NEITHER bar — it isn't a cost, it's a transfer to yourself
Like against like at last: what each roof burns per month. When the bars sit this close, the decision was never really financial.

Reading the model

Build both columns with named inputs and let a Data Table sweep the two assumptions that dominate: years you’ll stay (short tenancy → renting wins on buying costs alone; the crossover is typically 4–7 years) and appreciation vs investment return — the honest admission that the answer partly depends on two futures nobody knows. Sweep them, find where your crossover sits, and notice how often the grid says: close. That’s the model’s real gift — when the financials are within £100 a month either way, you’re released to decide on the things that actually differ: stability, flexibility, the garden, the landlord.

And when the bars aren’t close — a hot market where owning burns £600 more, a cheap one where it saves £400 — you’ve learned something slogans never could: the answer is local, personal and dated, which is why it belongs in your spreadsheet and not in anyone’s Twitter thread.

Unrecoverable costs, side by side, assumptions on the table. The most emotional question in money doesn’t get less emotional — it gets honest, which is the most a spreadsheet can do, and exactly what it’s for.