Before you call the architect, look under the house
If you own one of the older beach cottages in Hermosa Beach, the kind built in the 1950s or 60s a few blocks up from the Strand, and you're thinking about adding a second story, you've probably already had a contractor walk through and start talking about roof lines and setbacks. What most of those first conversations skip is what's happening below the floor and inside the walls you're about to build on top of. That matters more here than almost anywhere else in the county, because these houses have spent sixty or seventy years sitting a few blocks from salt water, breathing marine layer fog most mornings and never fully drying out in between.
Adding a second story changes the load on old framing and it changes how air moves through a house. Both of those things interact badly with moisture that's already been sitting quietly in a wall cavity or a crawlspace for decades. You want to know what's in there before you commit to building on top of it.
What the marine layer does to a house over decades
Hermosa Beach doesn't get much rain most years, but it gets fog and heavy night air almost constantly, especially in late spring and early summer. That moisture doesn't fall as precipitation, it just sits on surfaces and gets pulled into stucco, into old wood siding, into crawlspace soil that never quite dries between marine layer mornings. Older cottages built before modern vapor barriers were standard practice absorb this cycle year after year. It rarely causes a dramatic leak. It causes a slow, low-grade dampness that shows up as musty smell, soft baseboards, or a faint gray bloom on the underside of subfloor sheathing.
You can check for some of this yourself. Pull back a corner of carpet or look at exposed subfloor near exterior walls, especially on the ocean-facing side of the house. Look for dark staining or a chalky white residue on foundation walls in the crawlspace. Smell matters too. A closed-up crawlspace that smells sour or earthy beyond what you'd expect from sandy soil is telling you something.
More on this from What To Check Before Adding A Second Story To An Older Beach Cottage.
Crawlspaces and slab foundations near the sand
A lot of these older Hermosa Beach cottages sit on raised foundations with a crawlspace, which is good in one sense, since it keeps living space off the ground, but bad in another, since that crawlspace often has poor ventilation and sits close to a water table that's higher than people expect this close to the beach. Some newer additions or remodels on these same lots switched to slab foundations, which trap any moisture that was in the ground at pour time and give it nowhere to go.
Before you add a second story, get into the crawlspace with a flashlight, not just a contractor's quick glance. Check the vents aren't blocked by landscaping, sand drift, or a deck built over them at some point. If you can't get under the house easily, that's already useful information, because it means a mold inspector likely can't either without opening something up, and that's worth knowing before framing starts.
Why adding weight and height changes the moisture picture
A second story adds structural load, and older framing that was built to carry a single-story roof gets asked to do more. Contractors will check that math. What they won't necessarily check is whether the wall cavities they're about to build inside, or run new plumbing and electrical through, already have moisture damage that a structural inspection wouldn't flag. Old cottages here often have a mix of original 1950s framing and patchwork remodels from the 70s through the 90s, and each transition point, where new meets old, is a place water has historically found a way in.
A second story also changes airflow. Right now hot air escapes through gaps in an old roof. Once you build a conditioned second floor over that same footprint, you often trap humidity that used to vent freely, especially in a bathroom or kitchen wall that gets rebuilt as part of the addition. That's a mechanism worth understanding before the walls go up, not after.
Where this stops being a homeowner job
Looking, smelling, and pulling back a corner of flooring is reasonable homeowner work. Deciding whether a stain is old and dry or active and spreading is not something you can reliably judge by eye, especially in a coastal climate where wood can look fine and still be holding more moisture than it should. That's the point to bring in a mold inspection before permits are pulled, not after drywall goes up on a new second floor and covers whatever was underneath.
A mold inspection before a major addition is a small cost measured against tearing out new framing later because something behind an old wall kept growing after it got sealed in. In a town where nearly every older house has spent decades absorbing salt air and fog, that inspection is less about catching a disaster and more about knowing exactly what you're building on top of.