Design & Construction

Design & Construction

Creating an ADU is a multi-step process that requires focused follow-up. While it is possible to do this as an owner/contractor we highly recommend that you research and choose an experienced firm that specializes in ADU construction. This will prevent many headaches and save you unnecessary losses of time and money. An ADU construction firm should be familiar with your local regulations and zoning requirements. They should also have strong positive relationships with the city Building Department which will facilitate approval of planning and inspections.

1.  Planning your ADU project

Usually an ADU must be at least 150 square feet, about the size of a small bedroom or studio apartment, up to a maximum of 850 square feet. In some locales an ADU can be up to 1200 square feet or a fixed percentage of the size of the main house.


While most ADUs are single story there are some places which will allow a 2-story ADU in a neighborhood of multi-story buildings.


In general, building the largest ADU you can afford for your site will yield both more rent and a greater increase in property value which will mean a higher resale value.

ADUs must meet local planning code requirements, which include:

  • Parking
  • Height
  • Setbacks
  • Landscape
  • Architectural review
  • Maximum size of a unit

Other common provisions may include:

  • An owner-occupancy requirement
  • Dimensional and design standards to ensure neighborhood compatibility
  • Off-street parking requirements
  • Minimum lot sizes
  • Limits on the number of occupants or bedrooms

2.  Designing your ADU

You are usually not required to hire an architect to design your ADU, but you must have someone experienced with ADU design to prepare your plans to submit for permitted construction. To obtain building permits you must either apply as an owner/builder or your contractor will handle the applications. Your building department will supply a detailed list of all the permit costs, including the costs for utility hook-ups.

When you are working with an experienced ADU construction firm they will most often have a variety of approved plans for you to consider as is or for your personal alteration.


  • Because ADUs are smaller than most family homes they typically rely on open floor plans to maximize the use of space. Using universal design principles will ensure accessibility for all regardless of age, size or mobility.
  • Both interior and exterior should use smooth and level flooring, doorways wide enough to accommodate wheelchairs, appliances that are accessible and which require low physical effort and landscaping which does not impede movement.
  • Most ADUs are being built with solar energy incorporated into the design.
  • Green design and materials address residents’ health and safety as well as environmental concerns
  • Here are a few sample designs for one and two bedroom ADUs under 800 square feet.
    View gallery.


3. ADU Construction

ADUs are either stick-built on site or factory built homes delivered to be assembled on site or completely assembled and delivered to the site. Factory built ADUs usually save both time and money. It is worth noting that most new home construction is at least partially factory built. Your choice will likely be determined by a combination of the requirements of your particular site as well as your budget.

Most experienced ADU contractors work with one or more firms which will create your ADU package. Your selection of a contractor will depend on what type of ADU you want to build as well as your careful investigation of the contractor.

  • Hiring a contractor

    Hire a local, licensed contractor. Get multiple estimates before choosing one. Pay attention to their level of experience in new construction, not just remodels. Specific experience building ADUs is extremely valuable.

    • Check their past work. Get references.
    • Check their insurance and bonding.
    • Create a legal contract with a detailed itemized scope of work.
    • Never pay up-front. Pay by completed, inspected items.
    • The contract must include your right to cancel.
    • Get written warranties for materials and workmanship.
    • Set timelines for completion and penalties for delays.
    • Get proof that all subcontractors and suppliers have been paid before your final payment to your contractor.

  • Building process

    There are many stages in any new construction. Many of these stages depend on a building inspector’s approval of the prior stage in order to proceed. 

    • Site Clearing & Building Pad Preparation
    • Foundation
    • Underground Utilities
    • Framing
    • Roof
    • Exterior Surfaces
    • Windows and Doors
    • Electrical and Plumbing
    • HVAC
    • Interior walls and floors
    • Cabinetry
    • Lighting
    • Appliances 

  • Tips for managing construction.

    Your general contractor is usually your project manager, but you must manage your contractor. Set up regular weekly meetings and if possible, onsite visits. If you are not local, have video walk throughs. 

    • Establish positive communication when things are going well so you will be able to rely on a positive relationship if you run into trouble. 
    • There are always unexpected issues that arise in construction. Be ready for them.

 4. Financing ADUs — Overview of financing options

  • Home equity loans or home equity lines of credit
  • Cash out refinancing your home loan. New FHA guidelines allow you to include 75% of the rental income of an ADU to qualify for an FHA or Fanie Mae loan.
  • Credit unions may have specific loan programs based on up to 125% of the increased property value of your ADU.
  • Local and state governments are promoting ADU construction and may provide grants and/or waive permitting fees. Sometimes they offer specific programs if you commit to renting your ADU for low or moderate income housing.
  • Borrow from your retirement account or the cash value in your life insurance policy. These loans often do not have to be repaid and are typically at very low interest rates.
  • For detailed info on financing your ADU visit the Casita Coalitions ADU Finance Guide.
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