Remodels are substantially cheaper than new, free-standing dwellings. A few interior modifications can create comfortable living spaces that are more than adequate for many purposes. An “efficiency kitchen” that does not include a full stove or oven is allowed in any home, in addition to a standard kitchen. Essentially, a sink, refrigerator, counter space, and small appliances that plug into a standard 110 V wall outlet can be added to a bedroom, den or living room to create a defined living space that is effectively independent from the main home. These areas must maintain interior access to the rest of the home, since they are not fully independent ADUs.

For more information about the efficiency kitchen option, see Types of ADUs.

If there is existing space in your home or an accessory structure that would be suitable for a Conversion ADU or JADU, construction and permitting costs could be substantially reduced as compared to building a New Construction ADU. A Conversion ADU must include a full bathroom, full kitchen, separate entrance, separate heat source and sound- and fire-wall separation from the primary home. A JADU must include a separate entrance and efficiency kitchen, but may share a bathroom with the primary home.

It may be simple and cost-effective to convert an existing garage to an ADU. Consult with a contractor regarding the condition of the existing structure to see if conversion is more feasible than new construction. Also, if your property is located in certain Coastal Zone areas, you must make sure that there is still adequate parking on site for the primary dwelling after the conversion.

If you need a detached ADU, you might consider a mobile home, modular home or prefab “kit” construction. Factory-built housing is acceptable if placed on a permanent foundation and connected to site utilities. Pre-fabricated units cut construction costs significantly. Requiring fewer building inspections, they are quicker to build and occupy. However, remember that detached ADUs must meet state requirements for solar PV, so the cost of PV panels should be factored into any detached ADU project.

See Online Resources for links to manufactured and prefab home dealers.

Smaller ADUs have much lower fees than larger ADUs. ADUs less than 750 square feet are exempt from both impact fees and most building permit review fees. These savings can add up. For example, the County fees for a new construction ADU of 750 square feet are about $25,000, compared to about $5,000 for a 640-square-foot ADU.

Parking. New Construction ADUs require one additional on-site parking space (with certain exceptions), but spaces may be arranged with up to three spaces in tandem. No more than half of the front yard area (usually the first 20 feet) can be filled in with parking spaces or walkways.

Development Standards. Design your ADU to meet development standards such as height and setbacks from the property line, as well as to utilize special standards for ADUs, such as 4-foot side and rear yard setbacks, or the 3-foot minimum separation allowed between ADUs and other structures. An ADU of up to 800 square feet can be approved even if it exceeds maximum lot coverage or floor area ratio. If constructing an ADU over a new garage, the garage may be eligible for special setback reductions. ADUs are potentially eligible for minor exceptions or variances to development standards, but these special approvals require notice to neighbors and review by the Planning Director and/or Zoning Administrator, adding time, cost and a degree of uncertainty to the approval.

Septic Systems. If you use a septic system, save construction and operating costs by locating the ADU nearby on the uphill side. Consult with County Environmental Health Services (scceh.com or 831-454-2022) early in the process to assess the adequacy of your existing septic system and the level of improvements required.

Sewer Systems. If your property is connected to the County sewer system, connection fees are charged for new construction ADUs. Conversion ADUs are exempt from connection fees and JADUs are exempt from connection fees unless built concurrently with a primary dwelling. All ADUs are subject to an annual assessment for sewer services that is collected as part of the parcel's annual assessment through the County Assessor's Office. Contact the Santa Cruz County Sanitation District with any questions about sewer (sccsdweb@santacruzcounty.us or 831-454-2160).