What happens to my SEO if I rebuild my website?

A URL accumulates the signals: the links pointing at it, the history of people choosing it in results, the record of what it has been about. The design sitting on top of that URL is not what is being ranked, and neither is the company that built it.

This is why the two versions of this question have the same answer. Rebuilding a site and changing agencies both feel like handing over something fragile. Neither event touches the ranking by itself. What touches the ranking is whether the addresses survive the handover.

Broken redirect mapping. Every URL that ranked has to resolve to the page that replaced it, one to one, with a permanent redirect. Not to the homepage, not to a category page, not to a plausible relative.

A redirect to the homepage is read as the old page being gone rather than moved, and the signals it carried do not transfer. Done across a site, that is the weekend where the traffic halves. The mapping work is dull, it is checkable in a spreadsheet before launch, and it is the difference between the two outcomes people attribute to luck.

Expect movement, then recovery. Google has to recrawl the changed pages and re-evaluate them, and during that window positions wobble in both directions. A dip in the first fortnight is not evidence of damage.

What tells you whether it went well is not the traffic line in week one. It is the coverage report: old URLs redirecting rather than erroring, new URLs indexed rather than excluded. Watch that first, and the traffic question answers itself a few weeks later.

Changing who maintains a site is not a ranking event. Changing the site is. If the new arrangement means a rebuild, the risk is the rebuild, and it is the redirect mapping inside it.

The parts worth securing before anything is rebuilt are unglamorous and easy to lose in a handover: the domain registration, the Search Console property, the analytics history, and the existing URL inventory exported while the old site is still up. An agency that cannot produce a full list of its own live URLs is telling you something about how the redirect mapping will go.