Managing an Investment Property in a Spreadsheet — and Where It Breaks

Almost every investor starts in a spreadsheet. It’s free, flexible, and for a single property it does the job well enough. The trouble is that spreadsheets scale linearly with effort while portfolios compound in complexity. Here’s how to build one that actually works — and the precise points where it stops.

Start with the cash flow tab, not the value tab

The mistake most people make is anchoring their spreadsheet on capital growth. Growth is unrealised and unknowable. Cash flow is what determines whether you can hold through a rate cycle.

Build a monthly cash flow tab with these rows: rent received, less property management (typically 6–10% plus letting fees, depending on your state and agent), council rates, water rates, insurance, strata or body corporate, repairs and maintenance, and loan interest. Keep principal separate — it’s not deductible and you’ll need that split at tax time.

At the bottom, calculate a running weekly cost. A property renting at $650/week on a $600,000 IO loan at 6.2% interest costs roughly $715/week to hold before depreciation ($600,000 × 0.062 ÷ 52). Seeing that $65/week shortfall in a cell is more useful than any headline yield figure.

Model serviceability the way your lender calculates it

If you plan to borrow again, you need to understand how lenders assess your capacity — and the metric that matters most now is your debt-to-income (DTI) ratio: total debt divided by gross annual income. APRA requires lenders to limit mortgage lending above 6x DTI to no more than 20% of new lending (effective February 2026). For investors building a portfolio, hitting that ceiling matters more than whether any single property covers its own repayments.

A column in your spreadsheet that tracks total debt (all loans, all properties) against your gross income will tell you your real borrowing headroom. Include it from property two onwards.

Lenders also run a serviceability test: they shade rental income to 75–80% of actual rent, then assess all your debt repayments at a rate 3% above the actual rate (the APRA buffer, maintained at 3% as of mid-2026). On a $600,000 loan at 6.5%, the assessed rate is approximately 9.5%. Add a column that recomputes your monthly repayment at that rate — it tells you what headroom you’re presenting to a lender’s credit team, not just what you’re paying now.

This is straightforward to build for one loan. It becomes error-prone the moment you have three loans across two lenders with different fixed and variable splits.

Build a depreciation schedule you can actually reconcile

Enter your quantity surveyor’s Division 40 and Division 43 figures as separate annual amounts. Division 43 (capital works at 2.5%) is straightforward. Division 40 (plant and equipment) declines each year — and if you bought the property second-hand after 9 May 2017, you cannot claim it on used assets at all.

A single column of static depreciation numbers will quietly overstate your deductions by year three. You need year-by-year values, and you need to zero out removed or expired assets. Most spreadsheets get this wrong.

Where it starts to break

Two properties in different structures

The first real crack appears when your second property sits in a different ownership structure. A property held personally and one held in an SMSF have completely different tax treatments, contribution rules, and cash flow constraints. You can’t simply sum a “total portfolio” row across them because the numbers aren’t comparable. You end up with separate files, and separate files never stay in sync.

Actuals versus forecasts

Your spreadsheet forecasts $650/week rent. The property was vacant for 18 days and the management fee changed. Now you have a forecast column and you’re manually pasting bank figures next to it. Do this across four properties for a full year and reconciliation becomes a weekend job you keep postponing.

Multiple currencies

Own something in the UK and something in Melbourne and your spreadsheet needs a live FX assumption baked into every conversion. Hardcode 1.92 GBP/AUD today and your equity position is wrong by the time you next open the file.

If you hold NZ properties: interest deductibility was fully restored from 1 April 2025, so a spreadsheet built during the phased restriction period (2021–2025) may have incorrect assumptions about which interest costs are deductible. Worth checking if you’ve been working from an old model.

Pre-purchase modelling without contaminating your real numbers

When you want to model a fifth acquisition, you copy the whole workbook, tweak it, and end up with Portfolio_FINAL_v2.xlsx. Version control by filename is how errors enter your decision-making.

What a spreadsheet is genuinely good for

None of this means abandon spreadsheets. For a single property, a well-built sheet is faster than any tool. For scratch calculations, back-of-envelope yield checks, and one-off sensitivity tests, nothing beats a blank cell.

The break point is roughly three properties, or two in different structures, or the first time you cross a currency. Past that, the maintenance cost of the spreadsheet exceeds its value.

Property Insights was built for exactly that transition: consolidated LVR, DTI tracking, and net yield across every structure, actuals tracked against forecasts, multi-currency holdings in their local denomination, and pre-purchase scenarios that never touch your live figures. You can keep the spreadsheet for what it’s good at.

Track your own portfolio free → prop-insights.app