Can Ethernet be installed in an existing house?
Yes. Ethernet is routinely added to finished homes using attic, basement, crawlspace or interior wall pathways. Small access holes are sometimes needed and are patched afterward; a full rewire is almost never necessary.
The pathways installers look for
From above: an accessible attic lets cable drop down interior wall cavities to any room below it. This is the easiest case and covers most single-story homes and the top floor of two-story homes.
From below: a basement or crawlspace lets cable come up into first-floor walls.
Between floors: the hard case. Getting from a basement to a second floor usually means finding a chase — a plumbing or HVAC run, a closet stack, or the space behind a soffit.
When walls cannot be opened
Slab homes with no attic sometimes need a surface raceway painted to match the wall, a baseboard route, or an exterior run brought back inside. These are good options, not failures — they just need to be agreed in advance.
What gets restored
Ask how access holes are handled. Most installers patch drywall; painting is often the homeowner's. A good pro will tell you exactly where they expect to cut before they start.