Diagnosing gutter overflow before paying for a full redesign

During heavy Pacific Northwest rain, where does your gutter overflow, and what did you find was actually causing it?

I’m trying to separate a routine cleaning problem from a drainage-layout problem before paying for unnecessary work. The useful details seem to be:

- Roof and gutter age
- Whether the gutters are clean when they overflow
- The exact location, such as corners, seams, the middle of a run, or below a valley
- Downspout size and spacing
- Whether extensions carry water well away from the foundation
- Any nearby steep roof section, dormer, or valley concentrating runoff

A clean gutter can still overflow if the downspout is undersized or partly blocked, the gutter has lost its pitch, or too much roof area drains into one short section. Poor discharge can also make an overflow look like a gutter problem when the real issue is water backing up at the outlet.

For those who’ve dealt with this, which fixes were worth prioritizing first: cleaning and flushing, correcting gutter pitch, adding or enlarging a downspout, installing a valley diverter, or improving the downspout extensions? I’m especially interested in affordable repairs that protect the fascia, siding, foundation area, and roof edge before considering a larger redesign.