Building an ADU in Los Angeles: A Step-by-Step Overview

Accessory Dwelling Units — commonly known as ADUs — have become one of the most popular and practical ways to add value to a Los Angeles property. Whether you're looking to house a family member, generate rental income, or simply make better use of your lot, an ADU is a smart investment. As an architect and contractor who has guided many homeowners through this process, here's what you need to know before breaking ground.

What is an ADU?

An ADU is a secondary residential unit on the same lot as a primary home. It can be attached to the main house, built as a garage conversion, or constructed as a fully detached structure. Los Angeles has significantly relaxed its ADU regulations in recent years, making now an ideal time to build.

There are several different options for ADU’s, depending on your lot size, budget and personal preference:

Detached ADU — A fully standalone structure separate from the main house. Built new on an unused portion of the lot. Most flexible in terms of size and layout, but requires a larger investment since it requires its own foundation, roof, and all new utilities.

Attached ADU — Built as an addition to the existing home, sharing at least one wall. Attached ADU’s have a smaller footprint and don’t consume as much of the yard/lot. They are also good for multigenerational living — easy interior access between units if desired. May save some costs in utility infrastructure.

Garage Conversion (ADU) — An existing attached or detached garage is converted into living space. Very cost-effective since the foundation and shell already exist. Los Angeles has favorable rules for these — you don't need to replace the parking space you're removing in most cases.

Above-Garage ADU — Living space built on top of an existing or new garage. Great for maximizing lot coverage when the footprint is limited. Requires structural upgrades to support the added load, but offers excellent separation from the main house.

Junior ADU (JADU) — Carved out of the existing square footage of the primary home — typically a bedroom with an added kitchenette and private entrance. Capped at 500 sq ft in California. The most affordable option since no new structure is built, but also the most limited in size and privacy.

Two-Story / Stacked ADU — A detached or attached ADU built over two floors to maximize square footage on a small footprint. Useful on narrow urban lots where you can't spread out but can build up.

The process, start to finish

• Feasibility assessment

  • We start by evaluating your lot size, zoning designation, setbacks, and utility connections to determine what type and size of ADU is possible. 

• Design and architecture

  • Once we know what's feasible, we develop architectural drawings, and 3D visualization — floor plans, elevations, site plans — that meet both your vision and LA's building standards. This phase typically takes approximately 2 weeks. 

  • M Design Studio has a range of pre-designed layouts to choose from. We can also tailor those designs for your project, or we can create something completely custom for you.

• Permit submittal to LADBS

  • We develop the architectural and structural plans as well as the Title 24 report (the energy calculation of your building) that LADBS requires.

  • Plans are submitted to the Los Angeles Department of Building and Safety (LADBS). Since 2020, ADU permits in LA are largely ministerial — meaning they can't be denied for discretionary reasons — which has dramatically sped up the process.

  • LADBS reviews your plans for code compliance. There may be one or two rounds of corrections. This phase can take anywhere from 4 weeks to several months depending on whether you use LADBS's standard or expedited review.

• Construction

  • Once permits are issued, construction begins. A straightforward detached ADU typically takes 4–6 months to build. 

  • M Design Studio is a one-stop shop that provides We manage all subcontractors, inspections, and coordination with the city throughout this phase. As a design-build firm, M Design Studio brings architecture and construction together under one roof, giving you a seamless experience and a single team accountable for your project start to finish. We are your single point of contact from first sketch to final inspection.

• Final inspection and certificate of occupancy

  • After construction, LADBS performs a final inspection. Once passed, you receive a Certificate of Occupancy — your ADU is officially legal and ready to use.

Typical costs and timeline

For a detached ADU in LA, expect to invest $350 – $400 per square foot depending on size, site conditions, and finishes. The full timeline from initial consultation to move-in typically runs 12 months. JADUs (Junior ADUs) and garage conversions converted from existing space are faster and less expensive.

Common mistakes to avoid

The biggest pitfalls we see: starting design without a feasibility check, underestimating utility upgrade costs, and choosing contractors who aren't familiar with LA's ADU-specific code requirements. Working with a design build firm eliminates most of these issues by keeping design intent and field execution in sync.

Every lot is different. If you're considering an ADU in the Los Angeles area, a 30-minute consultation can tell you exactly what's possible on your property and what to budget.