Understanding Backyard Apartments

By Brent Gallagher |

Backyard apartments, also known as accessory dwelling units (ADUs), are gaining popularity as a flexible housing solution. This guide explores their benefits, potential challenges, and considerations for those interested in adding one to their property.

A backyard apartment—officially known as an Accessories Dwelling Unit (ADU), granny flat, or carriage house—is a self-contained secondary residential unit located on the same lot as a primary single-family home. Building a detached backyard ADU typically costs between $150,000 and $350,000+ depending on size, site preparation, utility hookups, and local labor rates. They offer high financial returns through rental income while adding significant long-term property value.

1. Key Types of Backyard Apartments (ADUs)

Understanding the structural classification of ADUs helps determine construction complexity and permitting requirements:

ADU Type Description Average Cost Range Primary Advantages
Detached ADU Standalone cottage built separately from the main house in the backyard. $150,000 – $350,000+ Maximum privacy for occupants and homeowners; highest rental value.
Attached ADU Addition built onto the side or rear of the primary residence. $100,000 – $225,000 Lower utility connection costs; shares at least one existing exterior wall.
Garage Conversion Converting an existing attached or detached garage into living space. $60,000 – $130,000 Lowest initial construction cost; utilizes an existing foundation and framing.
Above-Garage ADU Second-story apartment built directly over an active or renovated garage. $150,000 – $275,000 Preserves yard footprint; ideal for narrow or small urban lots.

2. Typical Size, Layouts, and Construction Costs

Most municipalities cap the size of a detached ADU between 400 and 1,200 square feet, or a specific percentage of the primary home's size (often 50%):

  • Studio / Micro-ADU (300 – 450 sq. ft.): Open-concept room combining sleeping, living, and kitchenette space with a separate full bath. Best for single occupants, short-term rentals, or home offices.

  • 1-Bedroom ADU (500 – 750 sq. ft.): Includes a private bedroom, full kitchen, living room, full bathroom, and stacked washer/dryer hookups. This is the most common size for long-term rentals and aging family members.

  • 2-Bedroom ADU (800 – 1,200 sq. ft.): Accommodates small families or co-living roommates, maximizing monthly rental revenue.

Cost Breakdown Estimate for a 600 Sq. Ft. Detached ADU

  • Architectural Plans, Engineering, & Permits: $10,000 – $25,000

  • Site Preparation, Trenching, & Utility Connections: $15,000 – $35,000

  • Foundation & Framing: $40,000 – $80,000

  • Interior Finishes, Plumbing, Electrical, & HVAC: $60,000 – $110,000

  • Total Project Cost: $125,000 – $250,000 (averaging $250 – $400 per sq. ft.).

3. Zoning, Regulations, and Legal Hurdles

Before hiring a contractor or designer, verify local municipal land-use codes regarding the following requirements:

  1. Setback Rules: Most cities require detached ADUs to sit a minimum distance from property lines (typically 4 feet from rear and side lot lines).

  2. Owner-Occupancy Mandates: Some jurisdictions require the property owner to live in either the main house or the ADU. Many states (like California and Oregon) have temporarily or permanently eliminated owner-occupancy rules to encourage housing supply.

  3. Parking Requirements: Many modern regulations waive mandatory off-street parking requirements for ADUs if the property sits within half a mile of public transit.

  4. Utility Hookup Regulations: Determine whether your city permits tapping directly into the primary home’s existing water, sewer, and electrical service or requires separate utility meters.

4. Financing Options and ROI

Homeowners typically finance backyard ADUs through several common avenues:

  • Home Equity Line of Credit (HELOC) or Home Equity Loan: Leverages existing equity in the primary home with flexible draw terms.

  • Cash-Out Refinance: Replaces the primary mortgage with a larger loan, using the extra cash for construction.

  • Construction / Renovation Loans: Specialized loans based on the after-renovation value (ARV) of the property.

Return on Investment (ROI)

  • Rental Income: A well-located 1-bedroom ADU renting for $1,500 to $2,500 per month can yield $18,000 to $30,000 in gross annual income, often generating positive cash flow after loan service payments.

  • Property Valuation: Real estate appraisals suggest that a permitted, legally built ADU increases a property’s resale value by approximately 20% to 35%, depending on local market demand.