Set up one page and one folder. The page is the log: what was done, when, by whom, what it cost, and when it is next due. The folder holds the paper behind it. That is the whole system, and it takes about an hour to start.
What follows is the version that turns out to be useful years later, which is a slightly different thing from the version that feels satisfying to make.
Step one: write down what is already in the house
Before recording anything you do from now on, record what is there. One line each for the roof, the heating system, the air conditioning, the water heater, the electrical panel, the windows and any major appliance.
For each, note the make and model, the age or install year if you know it, and where the data plate is. Photograph the plate while you are there, because it is invariably in a position that requires a flashlight and a shoulder at an angle.
Where you do not know an age, write down your best estimate and mark it as an estimate. An honest guess with a question mark is more useful than a blank, and it tells a future reader how much confidence to place in it.
Step two: record each job in five fields
Date. What was done, in plain words. Who did it, with a phone number. What it cost. What it means for next time.
That last field is the one that makes the log a tool rather than an archive. "Serviced, technician noted the capacitor is aging, expect replacement within two years" is a sentence that will save an emergency call, and it is exactly the sort of remark that gets said at a door and forgotten by dinner.
Write the entry the same day. A log kept in arrears is a log that stops.
Step three: keep the paper that proves it
Invoices, permits and inspection sign-offs, warranty registrations and manufacturer documents, and photographs of anything that got covered up.
Photographs of open walls, of a repaired subfloor, of a foundation before backfilling, are the items people never think to take and always wish they had. They answer the question of what is behind a finished surface, which arises during a future repair and again during a sale.
Physical or digital both work. What matters is that it is one place rather than four, and that somebody other than you could find it.
Step four: put the recurring items on a calendar rather than in the log
The log records history. A calendar drives the future, and mixing them makes both worse.
Four annual reminders and a short monthly one will carry a house: heating service before the cold, cooling service before the heat, gutters after the leaves, and a spring walk-around. Set them once with a week of lead time so a booking can be made at a normal rate rather than during a rush.
What it is worth at a sale
Three specific things, and they are more concrete than sellers expect.
It answers the inspector. An inspection report raises questions about the age and condition of systems, and a buyer reading "roof, age unknown" negotiates differently from a buyer reading a dated invoice with a transferable warranty attached.
It supports the disclosure. Most states require a seller to disclose known defects and past repairs. Answering yes to a prior water event with a folder showing the cause, the remediation and the sign-off is a completely different conversation from answering yes and shrugging.
And it transfers coverage. Some manufacturer warranties transfer to a buyer within a window after a sale, occasionally for a small fee, and an unregistered or undocumented warranty transfers nothing.
What not to put in it
Two things, and both are worth a sentence because people include them out of thoroughness.
Not routine consumption: filter changes, light bulbs, batteries. A log that records every small act becomes tedious and stops within a season. Record the things somebody would want to know years later, which is roughly anything with a professional, a permit or a part number attached.
And not account numbers or access codes. The log is a document you will hand to a buyer or leave with a house, and it should be readable by a stranger without giving them anything.
What it is worth in the meantime
Two things, and they arrive sooner than the sale.
A warranty claim in year eight is decided on whether you can produce dated service records, and the log is where the manufacturer's question gets answered in a minute rather than an evening.
And it makes spending predictable. Once the ages of the major systems are written on one page, the next decade stops being a series of surprises. A water heater in year eleven and a roof in year twenty-two are not emergencies, they are a schedule, and a household looking at that page in January can decide which one to do this year rather than having the decision made for it on a Sunday in February.