ADU Engineering Requirements: What Your Building Department Will Ask For

An accessory dwelling unit (ADU) is a complete small home on the same lot as a main house: a backyard cottage, an apartment above a garage, a converted garage or a basement unit. Because it is a dwelling and not a shed, it is permitted much like a house. Most building departments want structural drawings, a site plan and energy compliance documents before they issue the permit.

This guide explains what each part of an ADU submittal usually covers, what changes between a new detached unit and a conversion, and where projects most often get stuck in plan review.

Which ADUs need an engineer?

Most U.S. jurisdictions adopt the International Residential Code (IRC) with local amendments. The IRC lets simple wood-frame houses be built from its prescriptive tables. ADUs often fall outside those tables, and that is when the code requires an engineered design. Common triggers are:

• Large window and door openings, or open floor plans that need beams and posts sized by calculation
• A second-floor unit over a garage, where the garage door wall has very little bracing
• High wind, heavy snow or high seismic areas
• Expansive clay, fill or sloping lots, where standard footings are not enough
• Conversions that remove or cut existing walls, rafters or joists

Even when the framing could follow the prescriptive tables, many cities require sealed structural sheets for any new dwelling unit, so read your jurisdiction’s ADU submittal checklist early.

Planning a 500, 600, 900 or 1,200 sq ft ADU?

Many state and local ADU rules cap a detached unit somewhere around 800 to 1,200 square feet, and some tie the limit to the size of the main house or the lot, so confirm your local maximum first. Here is how size usually plays out:

• 400 to 500 sq ft: a studio or one-bedroom unit, usually single-story on a slab. The simplest to design and permit.
• 600 sq ft: a one-bedroom with a full kitchen and bath. A common size for garage conversions with a small addition.
• 800 to 900 sq ft: two bedrooms. Open living areas often need a beam or two sized by calculation.
• 1,000 to 1,200 sq ft: two bedrooms and two baths, sometimes two stories or built over a garage. These need full lateral bracing design and careful site planning to meet setbacks.

Size alone rarely decides whether engineering is needed. Shape, openings, number of stories and site conditions do. If you already have a floor plan you like, we can engineer it for your lot and jurisdiction.

What an ADU permit set usually includes

Site plan

A scaled site plan shows property lines, the existing house, the new unit, setbacks, easements, lot coverage, parking where required and the routes for water, sewer and power. Many state ADU laws limit how strict local setback and parking rules can be, but the plan still has to show the numbers. See our site plans for permits service.

Architectural drawings

Floor plans, elevations and sections showing egress windows, smoke and carbon monoxide alarms, stair dimensions and ceiling heights.

Structural drawings and calculations

Foundation plan, framing plans, beam and header sizes, shear walls and hold-downs, connection details and the design loads used. For a garage conversion the engineer also checks whether the existing slab and footings can carry new walls, and how the garage door opening will be infilled. Our structural engineering page explains what is included.

Mechanical, electrical and plumbing

Some departments accept MEP notes on the architectural sheets for a small unit. Others want a panel load calculation, a sewer connection detail and an HVAC sizing report. A new kitchen and bathroom often push the main electrical panel past its capacity, so a load calculation early avoids a surprise later. See MEP engineering.

Energy compliance

Insulation values, window performance and equipment efficiency must meet the adopted energy code. This is usually a compliance form or report submitted with the plans.

Garage and basement conversions: the extra checks

• Fire separation. If part of the garage stays a garage, the wall and ceiling between it and the living space need fire-resistant construction, and the connecting door must meet the code.
• Floor and moisture. Garage slabs usually slope toward the door and often lack a vapor barrier, so the new floor build-up and insulation need a detail.
• Ceiling height and escape openings. Basements need enough clear height and an emergency escape opening in every sleeping room.
• Existing framing. Removing a post, cutting a joist or adding a dormer changes how loads reach the foundation, so the engineer traces the load path all the way down.

Where ADU permits get delayed

The comments we see most often from plan reviewers are missing setback dimensions, beams without calculations, no shear wall or hold-down details, and no electrical load calculation. A complete first submittal is the fastest route to approval, which is why we check the jurisdiction’s checklist before drawing anything.

How Residential PE helps

Send your survey, sketch or existing drawings through the quote form. A licensed engineer reviews them, confirms what your building department requires, and quotes one fixed fee for the structural, site and MEP sheets you actually need. Revisions requested by the plan reviewer are included until the permit is approved.

Common questions