Arlington | Secondary Units and Duplexes

Arlington Secondary Units and Duplexes: Family Units, Garage Apartments and Duplex Lots

Arlington's citywide code allows three kinds of second home on a lot: the secondary living unit, the garage apartment and caretaker's quarters. Each has its own districts and limits, and the first one may not be rented. The accessory dwelling unit (ADU), the small second home that many cities allow behind a house, appears in Arlington's code only in the downtown form-based districts adopted in 2026. This page sets out each type and the rules for a duplex. The Arlington overview gives the short version.

The Four Kinds of Second Home

Source: Arlington UDC, Table 3.3-1, Sections 3.3.5 and 3.3.6, Section 12.3.5, Table 5.4-1; Article 13, Section 13.1.7.C.3 and Table 13.1.9-1.

The Secondary Living Unit: a Family Home

Section 12.3.5.N defines it as "a second single-family residential living unit located on the same lot as the primary living unit, which provides independent provisions for living/sleeping, cooking, and sanitation." The next sentences set the limit that matters most: "The secondary living unit is only for use by family members or relatives. This unit may either be attached or detached from the primary unit, but it shall not be rented or leased." If it is attached, it needs "a separate independent entrance not connected to the primary structure," and "a mobile home or HUD-Code manufactured home shall not be considered an accessory secondary living unit."

Section 3.3.6.L adds three conditions:

• Same standards as the house. The unit "must meet all standards as applied to the primary structure except the requirement for minimum gross living area, the requirement for a garage or carport, and the off-street parking requirements." So it needs no garage and no parking space.
• Size by rooms. It "must not contain more than one bedroom, more than one kitchen, or more than one bathroom."
• One per lot. "Only one secondary living unit is allowed per lot," and it counts toward the cap on accessory buildings in Table 3.3-4: three on a lot under one acre, five on one acre or more.

Section 4.1.1 also says "the density calculation does not include secondary residential units," so a secondary living unit does not use up the district's homes-per-acre limit. For size, a detached unit falls under the general rule for accessory structures in Section 3.3.5: its floor area "shall not exceed 50 percent of the floor area of the principal structure," and all buildings together must stay within the district's lot coverage.

Where the Secondary Living Unit Is Allowed

Table 3.3-1 marks the secondary living unit "P*" in RE, RS-20, RS-15 and VG, "S*" in RS-7.2, RS-5 and RM-12, and leaves RMF-22 blank. In practice the large-lot districts allow it by right, and most of the city's ordinary single-family neighborhoods, the ones zoned RS-7.2 and RS-5, need a specific use permit (SUP). An SUP is a City Council ordinance after a Planning and Zoning Commission hearing, with mailed notice to neighbors within 200 feet. The fee schedule effective October 1, 2026 charges $2,400 for an SUP on less than one acre.

The code is not consistent here. Section 3.3.6.L opens with "In all districts, this use shall be permitted under the following conditions," which reads as if no permit were needed anywhere. Section 3.1.2 says the letters in the table decide whether a use needs a permit, and that "a blank cell indicates that the use is prohibited." Section 3.3.6 is the list of extra standards that the table's asterisk points to, so the table controls whether you need an SUP. Confirm with the Zoning Administrator before you apply.

Rules for Any Accessory Building

A detached secondary living unit or garage apartment is also an accessory building. Section 3.3.5 sets the general limits:

• Height. "Accessory structures shall be limited to a maximum height of 24 feet," measured for accessory structures as "the height of the tallest wall" (Section 4.1.7.B).
• Timing. An accessory structure may not be built before construction of the main building starts, and "shall not be used until the construction of the primary structure is complete."
• Location. It must be on the same lot as the main building and may not sit in a platted or recorded easement.
• Ownership. "Accessory uses or structures shall be owned or operated by the same person as the principal use or structure."
• Setbacks. Section 3.3.6.A.3 keeps an accessory building five feet from side and rear lot lines, 15 feet from a street side on a corner lot (10 feet with a screening fence), and 20 feet from a rear line on an arterial or collector street. On lots in or next to single-family districts, a wall taller than eight feet needs two more feet of setback, and "in no case shall the highest point of the roof of the accessory building exceed the height of the primary structure."
• Materials. Under Table 3.3-3, a building of 550 square feet or more must use the exterior materials required for houses in Section 5.5.3.C.5, and its appearance "shall be architecturally compatible with the primary residence."

Garage Apartments and Caretaker's Quarters

A garage apartment is "a dwelling unit designed or constructed as part of a private garage" (Section 12.3.5.H). Table 3.3-1 allows it by right in RM-12 and RMF-22, and Table 3.3-2 allows it in DB, NMU and RMU. The code sets no rule on who may live in it. Section 3.3.6.F says it "shall be counted as a dwelling unit for purposes of calculating density on the site," and Table 5.4-1 asks for "1 per dwelling unit" in parking. Ordinance No. 26-019 rewrote Section 3.3.6.F to drop DB, which it made obsolete, so the 2026 text reads "In the RM-12, RMF-22, NMU, and RMU districts."

Caretaker's quarters are for "an employee or owner who looks after or takes charge of goods or property." The unit "shall be either inside or attached to a main structure by a common wall" (Section 12.3.5.C). Table 3.3-1 lists it in RE and VG. Under Section 3.3.6.C it must face the side or rear yard, may have no more than one bedroom, one kitchen and one bathroom, and "the owner shall sign and record restrictive covenants limiting use of the accessory dwelling unit prior to the issuance of a building permit."

Downtown Accessory Dwelling Units (2026)

The form-based code adopted by Ordinance No. 26-019 brings the ADU into Arlington's code, but only for land in a form-based district. Table 13.1.4-1 allows an "Accessory dwelling unit" in all four base sub-districts: TN, UN, CMU and Core. Section 13.1.7.C.3 says "ADUs shall be any dwelling unit 800 sq.ft. or under in conjunction with a primary dwelling unit on the same lot." An ADU "may be located within or attached to the main structure on the lot or in a detached structure on the lot or with a garage."

The ADU "shall be counted towards the maximum number of dwelling units permitted on the lot," but "only one (1) parking space shall be required per each ADU." In the Traditional Neighborhood sub-district, Table 13.1.6-1 requires "1,500 SF of lot area required for each dwelling unit" and treats accessory units as dwelling units for that count, so a house plus an ADU needs 3,000 square feet of lot, by VanPlex arithmetic. Article 13 sets no family-only rule for the downtown ADU. The zoning districts page explains where the downtown district applies.

Duplexes

Section 12.3.1 defines a duplex as "a detached house on a single lot designed for and occupied exclusively as the residence of not more than two families, each living as an independent housekeeping unit." Table 3.1-1 allows it by right in RM-12 and RMF-22 only.

Source: Arlington UDC, Sections 2.2.7 and 2.2.8, Table 5.4-1 and Section 5.4.9.B.2.

One number needs care. Section 4.1.1 measures density by "dividing the number of dwelling units on a site by the gross acreage of the site, including dedicated rights-of-way, private streets, and open space." A 6,000 square foot lot is 0.138 of an acre, so two homes on it work out to 14.5 homes per acre, above the 12 allowed in RM-12, by VanPlex arithmetic. Two homes at 12 per acre need 7,260 square feet. In a new subdivision the streets and open space count toward the site, which lowers the figure. For a single infill lot, ask the Zoning Administrator how the City applies the density cap before you buy a 6,000 square foot RM-12 lot for a duplex.

Section 4.1.6.G lets the front setback for "permitted townhouses and duplexes" drop to 15 feet "when access is provided by a private access easement at the rear of the structure." The design rules for two-family homes in Section 5.5.3.E add that "no primary entrance of a dwelling unit shall be located on the second story," and that each entry needs a porch, stoop or courtyard of at least 20 square feet. Subsection E caps front-facing garages at 60 percent of the building width, while Subsection D, which also names two-family homes, sets 55 percent; the code does not say which controls, so design to 55.

The Garage Rule and Other Limits on Duplexes

Section 5.4.9.B.2 says "an enclosed two-car garage is required for all single-family detached, townhouse, and duplex dwellings," at least 390 square feet. "A garage is not required for dwellings constructed prior to January 31, 1984," and a garage "may not be converted to living space unless another two-car garage is constructed on the same lot," except that homes built before January 31, 1984 may convert one if a driveway of at least 20 feet is built. Table 5.4-1 asks for "2 per dwelling unit" for a duplex. The parking, setbacks and height page covers these rules in full.

Two other places allow duplexes. A small-lot subdivision under Section 5.5.6 may "allow duplexes on minimum 5,000 square feet and 50-foot lot width," up to 30 percent of the homes. And the downtown TN and UN sub-districts list the duplex, side by side or stacked, as a permitted use. Near the airport, Table 5.9-1 marks "Dwelling, Townhouse | Duplex | Single-Family Attached" as N, prohibited, in APO-2, APO-3 and APO-4.

Where to Go Next

Related on VanPlex: the Arlington overview, ADUs in Texas, and building codes for duplexes to fourplexes.

Frequently Asked Questions

Official sources (checked October 2026)