What are Detached Accessory Dwelling Units (DADUs)?
Detached Accessory Dwelling Units (DADUs) are small, secondary homes located on the same property as a primary residence but are separate structures. Unlike attached accessory dwelling units (ADUs), which are part of or connected to the main house, DADUs stand alone as independent buildings. They are also known by various other names, such as backyard cottages, carriage houses, granny flats, or secondary suites.
DADUs typically feature essential living amenities, including:
- —Kitchen: A fully functional kitchen equipped with appliances for cooking and food preparation.
- —Bathroom: A private bathroom with the necessary fixtures.
- —Living and Sleeping Area: Space for daily living activities and sleeping quarters.
These units offer a versatile solution for accommodating extended family members, providing rental income, or serving as guest accommodations, while also enhancing the property's functionality and value.
Key Characteristics of DADUs:
- —Detached Structure: Unlike ADUs that are attached to the main house, DADUs are completely separate buildings, offering more privacy and independence for occupants.
- —Size Limitations: DADUs are typically smaller than the main residence, often ranging from a few hundred to about a thousand square feet. Local regulations usually dictate the maximum allowable size.
- —Independent Living: DADUs are designed to be self-sufficient, containing all the amenities needed for independent living, similar to a full-sized home but on a smaller scale.
- —Varied Uses: These units can serve multiple purposes, from housing relatives or guests to providing rental income or creating a home office or studio space.
- —Architectural Flexibility: DADUs can be customized to match the architectural style of the primary residence or to introduce a new aesthetic element to the property.
Benefits of DADUs:
- —Additional Income: Homeowners can rent out the DADU, generating extra income.
- —Family Accommodation: They provide a convenient housing solution for extended family members or aging parents, often referred to as "granny flats."
- —Property Value: Adding a DADU can increase the overall value of the property.
- —Urban Density: DADUs offer a way to increase housing density in urban areas without the need for large-scale apartment buildings.
- —Flexible Use: They can adapt to various needs over time, such as transitioning from a rental unit to a home office or guest house.
Regulatory Considerations for DADUs:
The construction and use of DADUs are subject to local zoning and building regulations, which can include:
- —Permitting: Obtaining the necessary permits and approvals before construction.
- —Setback Requirements: Adhering to rules about how close the DADU can be to property lines and other structures.
- —Size and Height Limits: Ensuring the DADU conforms to maximum size and height restrictions set by local codes.
- —Design Compatibility: Following guidelines to ensure the DADU is aesthetically compatible with the primary residence and the neighborhood.
By understanding these principles and regulations, homeowners can effectively incorporate DADUs into their properties, creating versatile and valuable living spaces that meet a variety of needs.
DESIGN GUIDELINES
Massing & Form
- —The footprint of an outbuilding should not exceed 750 square feet, except in the case of lots exceeding 10,000 square feet. In those cases, the footprint shall not exceed 1000 square feet.
- —Ridge heights shall not exceed 25’ from the existing grade for interior lots and shall not exceed the height of the primary dwelling for corner lots. The height of the historic building shall be determined based on the historic building and not ridge raises or tall additions. While an outbuilding may have a ridge height taller than the primary building for interior lots, a full two-story outbuilding is only appropriate behind a two-story primary building.
- —Maximum foundation height shall not exceed one foot from the existing grade on the corner of the building that sits on the highest area of the existing grade. (Grade may need to be adjusted for water runoff but should not be built up for the sole purpose of increasing building height.)
- —On outbuildings behind primary buildings that are one or one and one-half stories, the wall heights of an outbuilding shall not exceed twelve feet, and for an outbuilding behind a primary building that is two or more stories, the wall heights of an outbuilding shall not exceed 17’ from the existing grade as measured from the top of the finished floor/slab. Measurements shall be taken from the top of the finished floor/slab to the ridge or to where the sidewall and the roof intersect, regardless of whether the soffits are of an open or closed design.
- —The roof slope of the outbuilding shall be at least 4/12.
- —Stairs to another level, not counting stairs to access a porch or stoop, should be interior.
- —Eaves should not extend more than two feet.
Siting & Setbacks
- —Generally, new outbuildings should be placed in rear yards, close to the rear property line, or in the original location of a historic accessory structure.
- —In many cases, outbuildings may be as close as 5’ to a rear or side property line, with the following exceptions: On corner lots, the outbuilding should be a minimum of 10’ from the street-side property line or 20’ if the garage doors face the side street. On double-frontage lots, the rear setback should match the historic context on the secondary street. If there is no context, it should be a minimum of 10’ from the rear property line or 20’ if the garage doors face the rear. On lots where a rear property line abuts a side-property line and there is no rear alley to separate the two properties, the rear setback should be a minimum of 10’.
- —On corner lots, the outbuilding should be a minimum of 10’ from the street-side property line or 20’ if the garage doors face the side street.
- —On double-frontage lots, the rear setback should match the historic context on the secondary street. If there is no context, it should be a minimum of 10’ from the rear property line or 20’ if the garage doors face the rear.
- —On lots where a rear property line abuts a side-property line and there is no rear alley to separate the two properties, the rear setback should be a minimum of 10’.
- —An outbuilding should be a minimum of 6’ from any other building, even those that may be on neighboring properties.
- —When a setback determination is found to be appropriate, the “edge of the building” shall be considered the maximum of any protrusion beyond the footprint such as bays/oriels, balconies, awnings, hoods, and roof overhangs.
Add-On Features
Add-on features are available for outbuildings that will not be calculated into the maximum square footage but do need to meet setback requirements. Larger versions of the added features or features different from what is proposed in this section will be considered within the previous design requirements.
Hoods & Awnings
- —Hoods and awnings should not exceed 3’ in depth.
- —Hoods and awnings should only be located over windows and doors.
- —Width shall not exceed the opening it covers by more than 2’ on each side to allow for brackets and connections.
Stairwell Bay
- —All stairs should be enclosed. For forms that have a footprint of less than 500 square feet and that are 1.5 or 2 stories, a stairwell bay may be added.
- —No more than one per building.
- —A stairwell bay should not exceed 8’ wide and 4’ deep.
Enclosed Vestibule
- —Vestibules are fully or partially enclosed stoops.
- —They should not exceed 5’ wide and 4’ deep.
- —Should not exceed one-story.
- —No more than one per building.
Projecting Balcony
- —Should not have a cover.
- —Should not exceed 30 square feet.
- —No more than one per building.
Projecting Oriel
- —Should not exceed a depth of 2’.
- —No taller than 10’.
- —No wider than 10’.
- —No more than one per building.
Projecting Porch on the Ground Floor
- —Should not exceed the full width of the side of the building to which it is attached.
- —Should not exceed 6’ in depth.
- —Should be one-story only.
- —No more than one per building.
Roof Dormer
- —14’ wide total maximum.
- —Front-face of each dormer should be primarily glazing.
- —No more than one per roof plane.
- —Inset a minimum of 2’ from side walls and from the wall below.
- —Not appropriate for 2-story outbuildings.
Wall Dormer
- —14’ wide total maximum.
- —Front-face of each dormer should be primarily glazing.
- —No more than one per building.
- —Inset a minimum of 2’ from side walls.
- —Not appropriate for 2-story outbuildings.
Detached Accessory Dwelling Units offer a flexible and sustainable housing solution, accommodating diverse living arrangements while enhancing property value and neighborhood vitality. By adhering to thoughtful design principles and navigating the regulatory landscape with care, homeowners can realize the potential of DADUs as functional, beautiful, and harmonious additions to their properties. Whether as a cozy guest retreat, a home office, or a rental unit, DADUs represent a promising avenue for innovation in residential architecture and urban planning.
Disclosure
The information provided in this blog post is intended for general informational purposes only. While every effort has been made to ensure the accuracy and completeness of the content, zoning regulations are subject to change, and their interpretation may vary. Therefore, this post may not capture all nuances or recent updates.
Readers are advised to consult the Metropolitan Government of Nashville and Davidson County’s official zoning documentation or contact local zoning authorities for the most current and detailed information specific to their property or project. The author assumes no responsibility for any errors or omissions or for actions taken based on the information provided herein.
Resources
ORDINANCE NO. BL2011-900
NCZO for Turn-of-the-20th Century Districts Part I
NCZO for Mid-Century District
Belmont Hillsboro NCZO
Hillsboro West End NCZO