The cost to build an Accessory Dwelling Unit (ADU) typically ranges from $100,000 to $300,000, depending on the size, design, location, and type of build. On average, ADU construction costs $200–$400 per square foot, with smaller units often costing more per square foot due to fixed expenses like permits and utilities. Total pricing includes hard costs (labor, materials, foundation, utilities), soft costs (plans, engineering, permits), and optional upgrades. Site conditions, city fees, and utility connections can add to the final cost, while garage conversions and prefab ADUs may cost 20–30% less than traditional builds.

Cost factors include:

• ADU type (detached, attached, garage conversion, prefab)

• Size and design complexity

• Labor, materials, and construction method

• Permits, plans, and engineering fees

• Site conditions and utility connections

Convert an attached garage into a fully functional, code-compliant Accessory Dwelling Unit (ADU) by remodeling the existing structure without expanding its footprint. This approach turns underused space into comfortable living quarters while minimizing construction time and costs.

Construction Scope:

• Interior framing, insulation, and drywall to meet energy and building codes

• Floor leveling and upgraded ceiling finishes for a polished interior

• Full plumbing, electrical, and HVAC integration for independent living

• Private entrance for added privacy and accessibility

• Code-compliant fire separation and safety upgrades between the main home and ADU

Advantages:

• More affordable than ground-up ADU construction

• Faster permitting and shorter build timeline

• Uses existing structure and utility connections

• Minimal impact on yard space and exterior layout

• Ideal for in-laws, guests, rental income, or a dedicated home office

Transform a standalone garage into a fully independent ADU with its own living, kitchen, and bathroom areas. This approach maximizes privacy, adds versatile living space, and can generate rental income. You will be using the existing structure to save time and cost.

Construction Scope:

• Full interior build-out including framing, insulation, drywall, flooring, and ceiling finishes

• Structural upgrades to foundation, walls, and roof for code compliance and durability

• Separate plumbing, electrical, and HVAC systems for independent living

• Private entrance with optional outdoor space, patio, or yard

• Fire separation and safety measures to meet local building codes

• Layout and design customization to fit bedrooms, living areas, and storage

Advantages:

• Maximum privacy from the main house

• More affordable than building a new detached ADU

• Flexible design options to suit families, guests, or rental units

• Faster permitting and construction timeline using the existing structure

• Ideal for in-law suites, guest housing, home offices, or long-term rentals

Add a second story above your existing garage to create a two-level ADU, keeping the garage functional below while providing a fully independent living space above. This solution maximizes property use and adds significant square footage without expanding the home’s footprint.

Construction Scope:

• Structural reinforcement of the existing garage to support a second story

• Framing, roofing, and exterior finishes for the new upper-level unit

• Installation of new stairs and a separate entrance for privacy

• Full interior finish of the upper ADU, including kitchen, bathroom, living area, and bedroom(s)

• Integration of plumbing, electrical, and HVAC systems for independent operation

• Compliance with building codes, fire separation, and safety standards

Advantages:

• Adds significant living space without enlarging the property footprint

• Maintains the garage’s original function below

• Increases property value and rental potential

• Provides flexible options for family members, guests, or long-term rental income

• Offers privacy with a fully separate living unit above the garage

Setbacks, height limits, and lot coverage are zoning regulations that determine where and how large you can build an ADU on your property. These rules ensure your ADU fits safely within your lot and doesn’t negatively impact neighbors or the surrounding neighborhood. Understanding these limits early is crucial for designing a compliant ADU.

Key considerations include:

• Setbacks: Minimum distances your ADU must maintain from property lines, often 4 feet or less for side yards, depending on your city. This affects where the ADU can be positioned on your lot.

• Height Limits: Maximum allowed height for your ADU, usually 16–18 feet for single-story units, to maintain neighborhood character and comply with residential zoning.

• Lot Coverage: The total portion of your property that can be built on. Most areas allow ADUs to occupy up to 50% of the primary residence’s size, or a maximum of 1,200 sq ft, whichever is smaller.

• Integration with Existing Structures: Attached ADUs may share walls with your main house, while detached units must meet additional spacing and fire separation requirements.

• Permitting Impact: Your design must comply with these limits to receive approval. Violations can delay permits or require design changes.

By carefully planning setbacks, height, and lot coverage, you can maximize usable ADU space while staying fully compliant with local building codes.

Whether an ADU requires separate utility meters depends on local regulations, the type of ADU, and how it will be used. Some cities encourage separate meters for billing and safety, while others allow ADUs to share utilities with the main house.

Key points to know:

• Electricity: Detached ADUs often have their own meter, but attached units or garage conversions can sometimes share the main house’s meter if allowed.

• Water and Sewer: Local codes may require separate connections for billing or usage tracking, especially if the ADU will be rented.

• Gas: Similar to electricity, some ADUs share the main home’s gas line, but a separate meter may be recommended for safety and billing.

• Benefits of Separate Meters:

  • Clear utility billing for tenants or family members

  • Easier long-term management and rental arrangements

  • Can prevent disputes over shared utility costs

• Shared Utilities:

  • Typically allowed for family use ADUs where the unit isn’t rented

  • May reduce upfront installation costs but could complicate rental billing

Planning utilities early in the design phase ensures your ADU is fully functional, code-compliant, and cost-efficient.