Design the relationship between the two homes.

The first design problem is not the floor plan. It is how the new residence shares the property with the existing house. Consider the approach from the street, the placement of entries, the view from each living space, and the outdoor areas each household will use.

Windows that look attractive in isolation may face a neighbor or the main home's private space. A terrace may compete with a service route. Work through these relationships before committing to the building outline. Our ADU layout guide starts with everyday use rather than a fixed template.

The build starts outside the footprint.

A backyard ADU contractor needs to understand how the job will physically happen. Measure the access available for deliveries and equipment. Identify existing paving, fences, trees, drainage features, and the areas that may need protection or restoration.

Ask where power, water, and sewer will reach the new dwelling and who confirms their requirements. A short-looking line on a drawing can pass through difficult existing conditions. The proposal should explain what connection work is included and what still requires investigation.

  • Construction access and material staging.
  • Site preparation and existing features affected by work.
  • Utility routes, equipment locations, and provider coordination.
  • Outdoor circulation, privacy, and landscape restoration.

Site-built or prefab: compare the complete job.

A prefab or modular ADU may package part of the construction differently, but the property still needs a site-specific solution. Compare foundations, delivery or lifting access, service connections, permit responsibilities, and finishing work alongside the manufactured building price.

Site-built construction may allow different design and access options. The right choice depends on the lot, layout, construction method, and total scope—not the smallest advertised building number. Our ADU construction cost guide explains how to compare proposals with the same inclusions.

Make it comfortable for daily living.

Think about acoustics, daylight, ventilation, kitchen work surfaces, laundry, and storage from the beginning. Mechanical equipment should have a planned location, not consume a useful corner late in the design. Furniture should fit with doors open and someone moving through the room.

For family living, discuss present needs and future flexibility with the design team. Entry transitions, bathroom layout, and clear circulation can be important decisions without turning the plan into an oversized home.

Prepare a detached ADU project brief.

Collect a rough site sketch, photos from the access route, intended occupancy, and a short list of priorities. If you already have plans, include their date and review status. Then explore the Costa Mesa approval process and our construction approach.

Local source: City of Costa Mesa ADU resources. Check the current City instructions for your address and proposed work.