You are about to make a major change to the site (a big update, a migration, a redesign) and you do not want visitors landing on half-finished work. The solution seems obvious: put up a maintenance page. What is less obvious is that there is a correct way to do it and a way that, invisibly, tells Google your site stopped existing.
The mistake you cannot see but pays for dearly
When you turn on a maintenance page without configuring anything else, in many cases the server responds with the status code that normally means everything went fine (a successful response), simply showing different text. The visitor sees the maintenance notice and notices nothing odd. But a search engine crawling that address at that moment reads something entirely different: that the page now has this content permanently, and it can start replacing what it had indexed with the maintenance notice.
If maintenance lasts a few hours, nothing serious probably happens. If it lasts several days with that incorrect setup, you run a real risk of the site's pages getting reindexed showing the maintenance notice instead of the real content, and reversing that is not instant.
The correct code: 503
The right way to communicate temporary maintenance is with the 503 status code, which means exactly that: "the service is unavailable right now, but it is temporary". It is the signal that tells a crawler "do not reindex me now, try again later", instead of "this changed forever".
With 503 correctly configured, a crawler passing through during maintenance does not replace the indexed content: it simply notes it could not access it at that moment, and tries again later. The rankings you already had stay protected for the duration of the maintenance window.
One extra detail worth adding: the same response can include an indication of roughly how long the interruption will last, so the crawler knows when it is worth coming back instead of retrying too early.
What to show the visitor in the meantime
Getting the technical code right does not mean the page has to be ugly or empty. A good maintenance page has three elements: a clear message that this is temporary and the site has not disappeared, an honest estimate of how long it might take if you have one, and some way to get in touch or follow up if the visitor needs something urgently.
What is worth avoiding is a completely blank page or a generic error with no explanation: it creates the impression that the site is broken, not that work is in progress, and that affects how anyone arriving at that exact moment perceives it.
How long is reasonable to leave it up
For small changes (a plugin update, a configuration tweak) maintenance should last minutes, not hours. For big jobs (a full migration, an extensive redesign) leaving it longer can be justified, but with 503 correctly configured that window stops being a ranking risk, as long as it does not stretch on indefinitely.
As a practical reference: if the work is going to take more than a day, it is worth reconsidering whether the entire site genuinely needs to come down, or whether it can be done in parts, leaving the rest running while you work on one specific section.
If the work runs longer than planned, update the visible message with a new estimate instead of leaving the original, now-expired date on display. An outdated notice breeds more distrust than an honest one that admits the delay.
Treat maintenance mode as a tool you reach for deliberately, not a default you leave switched on between changes just in case. Every extra hour it stays up is an extra hour of visitors seeing a notice instead of your actual site.
How to turn it on without locking yourself out
A frequent mistake when setting up a maintenance page is leaving even the person doing the work without access. The standard approach is to configure an exception for your own address, so you see the real site while everyone else sees the notice, letting you verify the work in progress without having to switch maintenance off every time you need to check something.
Alternatives to taking down the whole site
Not every job justifies putting the entire site into maintenance. If the change affects only one specific section (part of the catalogue, a specific form), consider whether it makes sense to leave the rest of the site running normally and confine the notice to just that part, instead of cutting off access to everything for a narrow piece of work.
For work that genuinely requires taking everything down (a database migration, say, where content could end up inconsistent mid-process), full maintenance is the right call. The practical rule is simple: the narrower the work, the narrower the notice should be too.
And if the work can be scheduled, pick the site's quietest traffic window to do it. Even though a correct 503 protects rankings, there are still real visitors on the other end, and doing it during the lowest-activity period reduces how many people run into the notice instead of the content they came for.