Additions & ADUs
What does it cost to build an ADU in Pasadena?
By Michael Ward, owner of MKW Homes Inc
By Michael Ward, owner of MKW Homes Inc · Updated August 2026
The short answer
The type sets your starting price: a garage conversion is the cheapest, an attached unit sits in the middle, and detached new construction costs the most. After that, the biggest driver is how far water, sewer, and power have to run to the new space, and whether the lot needs grading.
Type sets the baseline
Three build types, three price bands. A garage conversion reuses the walls, roof, and slab, so you mostly pay for insulation, plumbing, and finishes. An attached unit shares a wall with your house and often taps existing utilities more easily. Detached new construction means a fresh foundation, framing, and its own utility runs, so it lands highest per square foot. Choose the type before you fall for a floor plan.
Site conditions decide the rest
Here is what surprises people. Two identical units on two lots can price thousands apart because of what sits underground. If the sewer main is far from the new unit, or your old clay lateral needs replacing to pass the permit, that trench costs real money. Same with running water and power across the yard. A sloped lot needs grading, a grading plan, and sometimes a retaining wall. A flat spot by the house is cheap. A hillside corner is not.
The mistake that blows budgets
The common mistake is pricing only the building and forgetting everything around it. California exempts ADUs under 750 square feet from impact fees and eased approvals, but that does not touch your site costs. People also skip the survey, design, and engineering, then get shocked at bid time. Build the budget in parts: permits and plans, site and utility work, the structure, then finishes.
What to ask a contractor
Ask three things before you sign. First, where does the sewer connect, and have you checked the condition of the lateral. Second, what does the site plan assume for grading and utility distance, and what happens to our price if those change. Third, is this a fixed bid or an allowance that can drift. A builder who has pulled ADU permits in Pasadena answers these fast. Vague replies on utilities and grading are your warning sign.
The one thing to remember
Type sets your starting number and the ground sets the rest. Decide between a garage conversion, attached, or detached unit first, then get a real look at your sewer, utility runs, and slope before you trust any price. The cheapest lot on paper is not always the cheapest to build on.
Follow-up questionsAnswered.
Is a garage conversion always the cheapest ADU?
Usually, because you keep the existing slab, walls, and roof. Not always, though. If the old foundation is failing or the garage sits on a slope, a conversion can creep up toward the cost of an attached build. Check the foundation first.
Did California ADU law make building cheaper in Pasadena?
It made approvals faster and exempts units under 750 square feet from impact fees, which helps. But the law cannot change your dirt. Utility distance, sewer condition, and slope still set most of the price. Easier permitting saves you time, not the actual cost of pouring concrete and framing.
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About the author
Michael Ward, owner of MKW Homes Inc
Michael is a Liverpool-trained builder with over 20 years in the trade, running one four-person crew across Pasadena and the San Gabriel Valley. California license CSLB #1074934, verifiable on the CSLB website. He writes these guides from what the work actually involves.
