Home / Garage Slab Lowering

EXCAVATION · UNDERPINNING · ADU

Your garage is short. We dig.

Most SF garage-to-ADU conversions die on one number: ceiling height. When the existing garage doesn’t clear legal habitable height, the floor comes down — excavation, underpinning of the existing foundation to your engineer’s plans, and a new slab with the drainage and vapor protection a living space requires.

Call (415) 687-3728 Get a free estimate →

FREE ON-SITE ESTIMATES
LICENSED & INSURED
PERMITS & INSPECTIONS HANDLED

SCOPE OF WORK

The dig-down, sequenced right

This is the highest-consequence concrete work on this site. It’s done in engineered stages or it isn’t done.

STAGE 01

Excavate in sections

The existing slab comes out and soil is excavated in an engineered sequence — never undermining the full foundation at once. Spoils are hauled through the garage door because on a 25-foot SF lot, that’s the only way out.

STAGE 02

Underpin the foundation

Existing footings are extended down in alternating pits per the structural drawings, each one inspected before the next opens. This is what keeps the house exactly where it was.

STAGE 03

The habitable slab

Sub-drain if the water table calls for it, vapor barrier, insulation where specified, steel, and the finished slab — flat enough for finish flooring, plumbed for the unit above it.

WHY THIS JOB EXISTS · SF ADU MATH

Inches of concrete, a unit of value

An SF garage that’s 6 inches too short is a storage room. The same garage after a dig-down is a legal rental unit on some of the most valuable residential land in the country.

⌂ The height problem is the whole problem

Legal habitable space has a minimum ceiling height, and old SF garages — built for Model As, not living rooms — routinely miss it once you account for new finished floors and ceiling assemblies.

Lowering the slab is usually the only direction to find those inches. Your architect and engineer set the target; we get the floor there.

≈ Dig-downs meet groundwater

Dig two feet down on a west-side sand lot or near old creek beds and you can meet seasonal groundwater. That’s a solvable problem — sub-slab drainage, sump, waterproofing — if it’s designed in, and a soaked ADU if it’s discovered after drywall.

We flag water risk in the estimate walk, before anyone’s committed to a budget.

§ Sequenced, permitted, inspected

Underpinning is permitted structural work with staged city inspections — excavation sequence, steel, and pours all get looked at. The engineer’s sequence drawings are the contract we build to.

If a contractor offers to dig your garage down without drawings, they’re offering to gamble your house. Decline.

COMMON QUESTIONS

Dig-down questions, answered straight

How much height can I gain?

Typically 12 to 24 inches, set by your engineer based on the existing foundation depth and soil. Deeper is possible with more underpinning. The architect works backward from required finished ceiling height; we build to that number.

What does garage slab lowering cost?

It’s a significant structural project — excavation, engineered underpinning, and a habitable-grade slab — and on most SF lots it lands in the tens of thousands, driven by depth, footprint, soil, and water. Against what a legal ADU adds in rent and resale, it’s routinely the best-returning concrete money in the city. Site visit first, then a fixed written number.

Will my house be safe during the work?

That’s what the sequence is for. Underpinning proceeds in small alternating sections so the house is always fully supported, each section inspected before the next opens. It’s slower than digging it all at once — deliberately.

Can I stay in the house during the dig-down?

Almost always yes. The work zone is the garage level; the noisy phases are demolition and excavation, and we’ll give you the schedule so you can plan around the loud weeks.

NO-COST · NO-PRESSURE

Get your free estimate

Tell us what’s cracking, settling, or waiting to be built. We’ll walk the site, measure it, and hand you a written price — free, and good whether you hire us or not.

Call now — (415) 687-3728

☎ Call (415) 687-3728 Free estimate