A $9,500 permit for stucco and dry rot repair at a single-family home on 20th Street is routine maintenance — and a small window into the upkeep demands on a neighborhood where roughly 42 percent of homes predate 1940.

At 1640 20th Street in Potrero Hill, about 200 square feet of stucco is coming off a stair wall. Permit No. 202607235457 — an over-the-counter alterations permit issued July 23 by the San Francisco Department of Building Inspection — authorizes removing the damaged cladding at the stair wall and lower windows, pulling back to the plywood beneath, repairing dry rot, and re-stuccoing to match the existing finish. Estimated cost for the single-family home: $9,500. Existing use: one-family dwelling. Proposed use: one-family dwelling. Nothing changes but the wall.

The permit record at a nearby property shows the same pattern: 255 A Potrero Avenue logged a repair described as "Replace dry rot at garage door. Patch stucco," at a cost of $4,000, per checkpermits.com. Two addresses, the same diagnosis.

Potrero Hill's housing stock earns that kind of attention. About 42 percent of housing units in the neighborhood were built before 1940, according to the Potrero View. San Francisco as a whole carries a median home build year of 1948 — third-oldest among the 100 largest U.S. counties, per the San Francisco Chronicle. Wood-frame construction with stucco cladding was standard for the era: durable, but not permanent. Stucco cracks; water works its way behind it; plywood softens; rot follows. Over-the-counter alterations account for roughly 76 percent of all building permits issued in San Francisco, according to data analyzed by OpenDataSanFrancisco — the volume a measure of how much of the city's housing stock is always, quietly, being held together.

The repair at 1640 20th Street is the kind of work that leaves no trace when it's done. The new stucco will be painted to match; the permit will close. The $9,500 is spent precisely so there is nothing left to see.