San Diego County is a great place to own a home and a hard place to keep paint on one. Inland, the sun bakes south- and west-facing walls for most of the year. On the coast, salt air and the marine layer keep surfaces damp every morning. Paint that isn't bonded to a clean, sound surface simply doesn't survive it.
The first cause of failure is dirt. Stucco and siding pick up a fine layer of dust, pollen, and chalking old paint that you can't always see. Paint applied over it bonds to the dust, not the wall, and starts to flake within a couple of seasons. A thorough pressure wash before painting is not optional.
The second cause is loose paint and bare wood left unprimed. Every flaking edge needs to be scraped and feathered, and every bare spot, whether wood, stucco patch, or rusted metal, needs the right primer before finish paint. Skipping primer is the single most common shortcut we see on houses we're asked to repaint early.
The third cause is the wrong product. Stucco moves with temperature and develops hairline cracks; an elastomeric or high-build acrylic coating bridges them, while a thin bargain paint just cracks with the wall. Wood trim needs a flexible, UV-resistant enamel. Matching the coating to the surface is half the job.
When a painter's estimate is much lower than the others, the difference is almost always prep. Ask what's included: washing, scraping, priming, caulking, and repairs. If the answer is vague, the finish will be short-lived. Done properly, an exterior in San Diego County should look good for eight to ten years.