The tool I needed didn't exist.
So I built it.

It started with my own tax return.

I own a rental in Auckland — a flatting setup, the kind where I live in the house with tenants and have to apportion shared costs between private use and rental use. The first year I tried to do it properly, I went straight to the source: IRD's own guidance, the floor-area method for shared rooms, the flat 50% rule for common areas, the interest-deductibility rules that were still being phased in at the time. I lost most of a weekend. Then another the year after, working through the same calculations from scratch.

So I did what software engineers do when something is repetitive: I went looking for a tool. What I found was underwhelming. Most property-management apps don't know what flatting is. The accounting platforms are built for businesses with bookkeepers, not someone with a single rental and an IR3 to file. The rules themselves — deductions, floor-area apportionment, interest limitation, depreciation, ring-fencing — are thorough but scattered across IRD pages, accountant blogs, and the occasional well-informed Reddit thread.

What I did find, though, was a community. Landlords on Facebook groups, on Reddit threads, on tenancy forums — quietly doing the same work I was. People managing their own returns with spreadsheets, cross-referencing IRD pages, double-checking each other's deduction logic, hoping they hadn't missed something that would come back to bite them.

That was the moment it clicked. I wasn't alone, and this wasn't a problem one more spreadsheet was going to solve.

So I built Zelvo — first for my own property, then, once it was actually saving me time, for anyone else still doing this by hand.