What is an ADU or Accessory Dwelling Unit?

An Accessory Dwelling Unit (ADU) is a separate dwelling unit on a lot shared with a primary dwelling. 

In Durham, in most urban zoning, an ADU can be constructed on a lot with a duplex for a total of 3 dwelling units.  An ADU can be placed within the primary structure, or in a separate accessory structure.  

Other names for ADUs include granny flat, in-law-suite, garage apartment, laneway house, alley house, backyard cottage, man cave and she shed.

What is an Accessory Building?

Accessory Buildings are separate structures on a lot that are subservient, or incidental, to a primary dwelling, in either form or function.  

This is a broad umbrella term that encompasses a wide variety of structures including barns, stables, (partially enclosed) carports, tree houses, garage units, garage apartments, pool-side bath houses, guest houses, woodworking shops, and tool sheds.

The primary distinction between Accessory Structure and Accessory Dwelling is the presence of cooking facilities. If your structure has a range oven or built-in cooktop, then the City of Durham will be treated as a dwelling.


Where do I find the “rules” for building an ADU in Durham?  

The UDO regulates the forms in Section 5.4.1 Accessory Structures, and Accessory Dwellings in Section 5.4.2 Accessory Dwellings. Although the code sounds wonky difficult, these sections are some of the shorter and more coherent sections of Durham’s code, and it is recommended you read them in their entirety.

Where are ADUs allowed?

ADUs are allowed on single-family and duplex homes in Durham.  In many suburban situations, a HomeOwners’ Association (HOA) covenants may prevent a homeowner from constructing an ADU. Due to recent zoning reforms, up to three ADUs are also permitted on lots with civic uses (such as churches, non-profit lodges, and community facilities).  ADUs do not count against density maximums, and are permitted on any residential lot, so long as the design can meet other dimensional standards.

What are some of the size restrictions to consider when designing an ADU?

The maximum size of an ADU in the urban portions of Durham is 800 square feet. The maximum height of an ADU in residential areas is 25 feet, measured to the midpoint of the roof.  Though it has not been tried to date, this enables some designs to have three floors of usable space, so long as the third floor is within the roofline.  

An accessory structure shall be clearly subordinate to the primary structure in aspects of size and purpose, and the structure shall be compatible with the primary structure in style, materials, roof form, and details. Interpreting compatibility can be subjective, but the design of most ADUs built in Durham to date match the primary structure in some way.

How does the size or shape of the lot affect design considerations?  

ADUs are subject to defined setbacks.  Setbacks are dimensional limitations placed on a property, based on zoning district, which restrict the location of various development elements. Typically, setbacks are from property lines, but can also be associated with things such as easements or structures.  In most urban districts, an accessory structure must be set back at least three feet from the side and rear lot lines. 

While setbacks are measured to the foundation of the structure. Other building elements have different setbacks, listed under  UDO Section 6.12.3 Encroachment into Required Yards:

  • Overhanging roofs have 2’ setback, 
  • Uncovered decks can extend 4‘ into any side and to within 4’ of the rear property line 
  • Parking, steps, fencing and trellises can be anywhere in the required setbacks.  
  • HVAC condensers must remain 3’ from the property lines. 

What are the basic zoning districts in Durham?  How can I figure out the zoning district for my lot?

You can identify your home’s information, including zoning, through .  After inserting your address, see the “Advanced” tab for the most critical information, including Zoning. Additional layers can be activated in the top left, under “Layer List”.  For schematic design, the most useful are often: Parcel Labels (which turns on lot dimensions), FEMA Flood Zones, Topographic Contours-2 feet, Zoning, and Stormwater Infrastructure.  These are the most likely layers to indicate opportunities and challenges to your project.

What are Firewalls?

A firewall is a partition or horizontal assembly (floor system) designed to inhibit or prevent the spread of fire. They are sometimes required between different building occupancies. For example, there are typically firewalls between two stores in the same commercial building.

Most ADUs are regulated through the IRC (Residential Building Code). This code requires firewalls between an attached accessory dwelling unit and the main house. See Section R302.3:

R302.3 Two-family dwellings. Dwelling units in two-family dwellings shall be separated from each other by wall and/or floor assemblies having not less than a 1-hour fire-resistance rating when tested in accordance with ASTM E119 or UL 263. Fire-resistance-rated floor-ceiling and wall assemblies shall extend to and be tight against the exterior wall, and wall assemblies shall extend from the foundation to the underside of the roof sheathing.


1. A fire-resistance rating of 1/2 hour shall be permitted in buildings equipped throughout with an automatic sprinkler system installed in accordance with NFPA 13.

2. Wall assemblies need not extend through attic spaces when the ceiling is protected by not less than 5/8-inch (15.9 mm) Type X gypsum board and an attic draft stop constructed as specified in Section R302.12.1 is provided above and along the wall assembly separating the dwellings. The structural framing supporting the ceiling shall also be protected by not less than 1/2-inch (12.7 mm) gypsum board or equivalent. R302.3.1 Supporting construction. When floor assemblies are required to be fire-resistance rated by Section R302.3, the supporting construction of such assemblies shall have an equal or greater fire-resistance rating.

This code section describes both firewall partitions (vertical) and floor systems (horizontal assemblies). The short summary is that 1-hour firewalls are required (without a sprinkler system) between the main house and accessory dwelling units. This is fairly straightforward if the ADU is attached laterally to the main house. The requirements get a little bit more complex if you stack units (all or part of an ADU over or under the main residence). This configuration requires that the walls that support the horizontal fire assemblies to also be firewalls.

While the jump from single family detached to duplex triggers additional code requirements in the International Residential Code (IRC), if an ADU is attached to a duplex, it will likely be reviewed as a triplex and thus push the project into the cost-prohibitive International Building Code (IBC), primarily intended for commercial use an larger apartment buildings.  Put another way the IRC covers 1-2 family structures.  Any structure with 3 or more units will be in the IBC.  The IBC requires larger stairwells, sprinklers, accessibility, etc.  Needless to say, if affordability is the goal, 1 and 2 family structures  are much simpler, less restrictive, and easier to attain affordability. 

An architect is the best professional to solve any firewall requirements your project may have.

Do ADUs require any off street parking spaces?  

In Durham ADUs do not require any additional off street parking spaces. 


How do I budget an ADU?

Often in construction budgeting is performed multiple times in a graduated process, getting more detailed and more precise at each stage.  You start broad and end specific.  A simple start is to ask what similar projects in Durham (or elsewhere) cost to build, and adjust for items that are known to be more or less expensive.  

While you can (and will try) to use a “dollar-per-sf” method, these are particularly misleading on ADUs.  Why? Because small structures squeeze out the cheap space (like bedrooms 3 & 4), while keeping the expensive space (like kitchens and bathrooms).  This raises the average cost-per-square-foot of a small home relative to a large home.  Where ADUs exist over garages (space that costs money to build but are calculated as zero square-feet in most circumstances), the data is even further skewed. At a certain point, the space is too small for cost-per-square-foot to be a useful metric.

To give useful price guidance, contractors will need to be provided construction drawings (CDs).  CDs would generally include a floor plan, foundation plan, elevations, electrical plan, roof plan, site plan and structural drawings,  drawn by an architect or draftsman, except the structural which is drawn by a structural engineer. You can have an architect create custom drawings or work from one of the many stock plans drawn from companies like Allison Ramsey Architects.   Once provided, a contractor will propose one of two contract types: 

  1. A fixed bid contract – a fixed price tied to a specific scope of work, variable for unforseen circumstances and deviations from scope, or
  2. A cost-plus contract – a variable price contract where you pay for the actual cost of construction plus either a % of work or a fixed fee for the services of the contractor.

There are costs and benefits to each.  Cost-plus tends to be easier (and quicker) to get in, harder to get out.  Fixed-bid is the reverse.  Cost-plus is usually open-book accounting, fixed-bid is closed. Ostensibly, fixed-bid locks in your price, but does this by shifting such risk to a contractor, where it may end up being more expensive than a cost-plus contract where the owner bears price risk.  Many contractors work exclusively with one type.  There is not a right or wrong way to do it.

After being provided with a preliminary budget, many contractors will work with you to identify cost saving opportunities in a value engineering cycle. 

Ask around for contractors that people like working with, you are likely to start hearing the same few names.  

Below is a non-scientific study of what members of Durham’s Builder’s Guild believes a 600 square-foot ADU-over-garage might cost to build, assuming builder-grade finishes and no unusual access or sitework.

Fees to the City ~ $7000 to ~$8000
Foundation work (600 sq. ft)$8,000
Plumbing (one bath and basic kitchen)$3000 to $7000
Electrical subcontractor$4000 to $8000
Framing subcontractor$10,000
Drywall and Paint contractor$10,000
HVAC$6000 to $8000
Flooring, Cabinets, counters$3000 to $8000
Hardware, Fixtures, Appliances$3000 to $5000
General Contractor Fees$30000 to 40000

How do I finance an ADU?

Currently, financing options for ADUs are limited.  To fund ADUs, most homeowners either self-finance (cash, savings, friends/family), or use equity in their home.  

There are three primary loan programs used to tap home equity:

  1. HELOCs (Home Equity Lines of Credit) allow you to write checks off of the equity in your home, and can go up and down as needed. While that flexibility can be extremely useful to many (particularly entrepreneurs with uneven income cycles), for those who are not experienced at money management or consistantly carry credit card debt, they can be dangerous.  
  2. HEILs (Home Equity Installment Loans) are becoming less common, but are essentially a 2nd mortgage, featuring fixed payments and no flexibility to park money the way the HELOC can.  
  3. Cash Out Refinancing involves terminating an existing loan (assuming you have one), and getting a new loan for a higher amount, allowing the homeowner to liquidate cash for the value difference between the two.  

Each option has a variety of benefits and drawbacks, and homeowners should discuss the details of their situation with a financial institution. Each of these equity finance options requires a lien of the home, which means the bank can foreclose and take the home in the event of non-payment.  

If a homeowner does not have the ability to self-finance, and lacks the equity in their home necessary to build an ADU, their options are limited.   Today’s lenders are not broadly experienced or interested in underwriting ADU’s, and there are no conforming debt programs in the United States specifically targeted at financing ADUs.  

There are a few interesting efforts to address this, but all are early stage and not currently in the market:

  1. As part of the Affordable Housing Bond, The City of Durham is working to establish a lending source for low-income homeowners for the development of ADUs. This is yet to be established. 
  2. Self-Help Credit Union (HQ in Durham) has a pilot ADU loan program in Los Angeles. If successful, they may bring the program to Durham in the future, but this is not currently being planned. 
  3. Additionally, a developer or builder of ADUs may consider custom financing options with specific homeowners. There are various start-ups pursuing such models, but none into functional existence as of yet.


How do I get a Permit for an ADU?

In Durham no special use permit is required for an ADU.  However, all improvements to real property require a building permit.  This is done administratively at the Development Review Office in the basement of City Hall.  

Plan reviews typically take 1-2 weeks, after which you can pick up the approved drawings and break ground.

Do I need a General Contractor?

In North Carolina a General Contractor is required on all jobs over $30,000 with an exemption for your personal property.  Thus, so long as you live in the primary dwelling, you can legally GC your own ADU project, subject to some restrictions. 

What ‘Impact Fees’ are levied on ADUs?  

Impact fees are typically broken into four levies: Sewer, Tap, Capital Facility Fee, $5500 per. 

As of February 2020, Accessory Building Permits cost $100 ($50 if there is no footing), + $50 Plan Review Fee, and Impact Fees, which vary by location.

Per the latest City Impact Fee Ordinance:       

  • Street Impact Fees of $293, $531 or $1,405 depending on which Economic Impact Zone the site is located in.
  • Open Space Fees of $222
  • Parks and Rec Facility Impact Fees of $425

The largest single fee is the sewer tap, on which more below.

Impact Fees for ADU Construction in DurhamAmount $
Impact Fees (Sewer, Tap, Capital Facilities)$5500
Building Permit for Accessory Buildings (usually $100 or $50 if no footing)$100
Plan Review Fee$50
Street Impact Fees ($293, $531, or $1405 depending on which Economic Impact Zone the site is located in)$531
Open Space Fee$222
Parks and Rec Facility Fees$425

*the sewer tap interpretation increased the impact fees 500%.

What compliance issues are there with sewer connection?

Both NC Residential Building Code and the State Administrative Code control sewer connections for ADUs.  Building Code requirements for ADUs are basic and identical to sewer requirements for all other single family dwellings. Since 2017, Durham Public Works has intermittently enforced an arcane part of the state administrative code that had previously been ignored. Functionally, this interpretation forced builders of detached ADUs to install a new sewer tap at great (and unnecessary) expense to the owner.  15A NCAC 02T prohibits connecting two separate buildings on the same parcel to the same sewer tap without a special permit issued by NC Department of Environmental Quality.   In February 2020, NCDEQ implemented a system meant to issue permits for a shared sewer line serving an ADU or an accessory structure or accessory building under narrow circumstances and subject to recording a deed restriction on the property.  As of April 2020, DEQ had received approximately 10 applications for Certificates of Compliance; no certificates had been issued.

Does an ADU require separate utilities?

Water service may be combined between units, sharing one tap.  This would require an owner to pay for or separately rebill water, which from a sustainability perspective is not ideal.  Studies show people who are aware of their resource usage tend to use less of that resource.  Still, sometimes the meter fees may be cost prohibitive, or there may be co-living situations where shared metering makes more sense, such as  when routing a new water service from meter to ADU may cause more site problems than connecting to existing adequate water service in the primary dwelling. 

NC Electrical Code does not require separate electric metering of ADUs.  Site constraints could favor serving a detached ADU from the primary dwelling’s electric service or from a separate electric service.  In most cases, NC Residential Building code requires that attached ADUs be served from the primary dwelling’s electric service.


What would a critical path look like for an ADU?

A critical path document graphically shows a schedule of tasks with start date, lengths and  dependencies.  They can be helpful in planning and project management. 

Inspections are generally handled by the general contractor or the specialty trade.  Usually inspections are required at footings, foundation, electrical, plumbing and mechanical rough in, framing, insulation, electrical, plumbing and mechanical final, building final, and engineering.  Engineering will ensure grade is proper and any sidewalks and driveway mouths are compliant.

After building final (otherwise known as Certificate of Occupancy or “C.O.”), you will have utilities turned on and be free to occupy the home.


All information is provided by pro bono professionals with the intent of educating non-professional citizens who are interested in adding housing to their community.  All construction and development has risk. This website and its contributors in no way warrant timely accuracy of the information and are indemnified against any and all liability related to its information.  Consult a professional to verify all information.