Going down costs more per square foot than going up
Above-grade construction is straightforward: you pour a foundation, frame the structure, and stack floors. Below-grade construction requires excavation, retaining walls, waterproofing, drainage, and ventilation systems that above-grade construction doesn't need. Industry rules of thumb put subterranean parking at $40,000 to $60,000 per stall in San Diego, compared to roughly $20,000 to $30,000 per stall for above-grade podium parking and $5,000 to $15,000 for surface or tuck-under.
That cost difference is per stall. On a project with 30 stalls, the choice between podium and subterranean can be a $1 million swing — frequently the difference between a feasible project and one that doesn't pencil.
The ramp consumes the lot — at the top and the bottom
Cars need a ramp to get below grade. A standard subterranean ramp at a 1:6 slope needs roughly 72 feet of run for a single level, plus a flat landing zone at the top before the ramp begins. On a 50-foot-wide infill lot, the ramp alone consumes most of one of the lot's two narrow dimensions.
But the ramp's footprint doesn't stop at the underground level. At its highest point — where the ramp opens to the surface — it requires vertical clearance for cars to enter and exit. That means the floor immediately above the ramp opening can't be rentable space. Whatever residential or commercial program would otherwise sit on the ground floor at that location simply can't exist there. The ramp claims a piece of the most valuable floor in the building twice: once underground for the ramp itself, and again above grade where the opening eliminates rentable area.
It's the same kind of efficiency loss elevator cores create on taller buildings — except it happens on the ground floor, where every square foot is the most valuable in the project.
A 7,000 square foot lot might dedicate 40% of its subterranean floor to circulation just to access a handful of stalls. At that point, you're paying premium subterranean costs to build mostly ramp — and giving up rentable area on the ground floor on top of it.
Shoring requirements explode when neighbors are close
San Diego's infill lots are typically narrow and bordered by existing buildings. Excavating to the property line next to a neighbor's foundation isn't just inadvisable — it requires shoring or underpinning systems that protect the neighbor's structure during construction.
These systems are expensive, and they require coordination, easements, or temporary construction agreements with adjacent property owners. On any project where the neighbors are close (which describes most of San Diego's infill), subterranean parking can require six-figure shoring costs before any actual parking is built. Sometimes neighbors won't agree at all, which can stall a project entirely.
On constrained sites, the shoring quote alone can exceed the cost of the entire above-grade parking alternative.
Water tables and soil
Coastal San Diego, much of Mission Valley, and parts of the older central neighborhoods sit on soils with high water tables — often within ten feet of the surface. Building below the water table requires permanent dewatering systems, more aggressive waterproofing, and sometimes structural design that has to resist hydrostatic pressure. Each of these adds cost. Some sites simply can't be built below grade economically at all.
A geotech report on a small infill lot can turn a "we'll do subterranean" plan into "we won't" in a single afternoon.
Construction sequencing extends the loan
Above-grade and at-grade parking can be built in parallel with the building above. Subterranean parking can't — you have to complete the excavation, retaining walls, and lower-level construction before the building above can rise. On a typical infill project, this can extend construction loan periods by three to six months. At current interest rates, that's real money, especially on a project where every month of carry costs erodes the margin.
Where subterranean does work
Subterranean parking isn't always wrong. It works on larger lots where the ramp footprint is a smaller share of the project, on sites where above-grade parking would block valuable street frontage or views, on high-rise projects where construction costs are already premium and the marginal cost of going down is less impactful, and on projects where the height envelope simply doesn't allow podium or above-grade alternatives. The Coastal Overlay Zone and Prop D height limits in particular can force subterranean in coastal projects where every above-grade foot is contested.
But on most infill lots — small parcels in non-coastal neighborhoods with neighbors close on either side — the alternatives win every time. Tuck-under parking, podium parking on larger lots, mechanical or stacked parking in tight situations, or AB 2097 parking exemption when the parcel is in a Transit Priority Area. Each of these solves the parking problem at a fraction of subterranean cost.
What this means for feasibility
When a feasibility analysis suggests "10 stalls below grade" on a small lot, the right next question isn't "can we make it cheaper?" — it's "do we need parking at all?" If AB 2097 applies, the answer might be no. If parking is required, the question becomes "what's the most economical configuration?" — and the answer is almost never below grade.
The code tells you how many stalls. Economics and geometry tell you where they actually go.