That musty smell during the walkthrough
You open a closet in the back bedroom and catch it, a flat, slightly sour smell that wasn't there in the living room. Or you notice the seller has a dehumidifier running quietly in the garage in the middle of a dry Los Angeles summer, which is a strange thing to need here unless something is feeding it. These small details are worth more than they seem. Los Angeles doesn't get the constant humidity of a Gulf Coast city, but it has its own damp pockets, and buyers who skip past them because "it never rains here" end up surprised later.
Why older LA housing stock hides water differently
A lot of the housing in neighborhoods like Eagle Rock, Mar Vista, or the older sections of the San Fernando Valley dates back to the 1920s through the 1950s. These homes often have cast iron or galvanized steel plumbing that's well past its intended life, original stucco that's been patched more than once, and crawl spaces that were never properly vented. None of that causes mold by itself, but it sets up the conditions. A slow leak behind a wall in a 1948 stucco bungalow can run for months without anyone noticing, because the wall assembly hides it well.
Newer construction and recent remodels bring a different risk. Flat or low-slope roofs, common on mid-century and contemporary builds across the Westside and in the hills, are less forgiving of poor flashing work. A roofer who didn't seal a penetration correctly won't show you a puddle, he'll show you a stain a year later, after a few winters of rain have worked their way in.
More on this from Reading List The Damp Questions Worth Asking Before You Buy In Los Angeles.
What the rainy season actually does here
Los Angeles gets most of its annual rainfall in a tight window, usually December through March, and a lot of it can arrive in a handful of storms rather than spread evenly. Homes that look bone dry in August can have real moisture problems that only show up after a wet winter. If you're buying in the dry months, which is most of the year, you're touring the house at its best. Ask the seller directly whether there's been any water intrusion during storms, and ask to see the attic and crawl space with a flashlight, not just the finished rooms.
Hillside properties around Silver Lake, the Hollywood Hills, or Pasadena's foothill neighborhoods have their own version of this problem. Retaining walls and hillside drainage systems that worked fine for decades can fail quietly, sending water toward a foundation during a single heavy winter. Ask when the drainage was last inspected, not just whether it's ever failed.
What you can check before you call anyone
Walk the perimeter of the house and look at the grading. Water should slope away from the foundation, not toward it. Check under sinks and around the water heater for any staining on the wood or cabinet base. Look at ceiling corners in bathrooms and any room below a bathroom for faint discoloration, even if it's been painted over. Open the attic access and look at the underside of the roof deck for dark staining or sagging insulation. A musty smell in a closet that doesn't have an obvious water source nearby, like an exterior wall closet, is worth asking about directly.
None of this requires special equipment. It requires time and a flashlight, and most sellers' agents will give you both if you ask.
When it stops being a homeowner job
If you find staining, smell something persistent, or the seller mentions a past leak that was "fixed," that's the point to bring in a mold inspection rather than guess. A visual check tells you something looks off. It doesn't tell you what's inside the wall cavity, whether the moisture source was actually resolved, or whether spores have spread into the HVAC system. An inspector can test air quality, check moisture levels inside walls without demolition, and give you a written report you can use in negotiations.
This matters more in Los Angeles than people expect, because so much of the housing stock has been through multiple owners and multiple remodels, each one potentially sealing a problem behind new drywall instead of fixing it. A clean-looking wall tells you about the paint job, not about what's behind it. For a purchase this size, a few hundred dollars for a proper inspection is a small cost next to the alternative of discovering a hidden problem after escrow closes.