Second Dwellings: What Cochise County Allows on One Parcel

The question comes in several forms — a casita for a parent, a guest house, somewhere for a caretaker, a second small place to rent. Cochise County has a specific answer, and it is more permissive than most people expect in one respect and considerably stricter in another.

What counts as an ADU

The county defines it as a self-contained living unit on the same lot or parcel as a single-family dwelling of greater square footage, including its own sleeping and sanitation facilities and possibly its own kitchen. The stated intent is to enhance housing options and affordability while preserving the character of residential and rural districts.

Three requirements gate the whole thing:

• An existing primary dwelling. The parcel must already have a single-family home. An ADU cannot come first, which rules out the popular plan of building the small place, living in it, and adding the main house later.
• Owner occupancy. The property owner must live in either the primary dwelling or the ADU as their primary residence. This is the requirement that stops a parcel becoming two rentals.
• Zoning compliance. ADUs are permitted where a single-family dwelling is permitted.

The permissive part: the county allows at least one attached and one detached ADU per parcel, and on lots one acre or larger, one additional detached ADU — subject to the rest of the standards, and excepting areas with high-noise or accident-potential zoning near airports. On rural acreage that is a genuinely useful allowance.

Size, and a discrepancy worth knowing about

Height is simple: the ADU must be equal to or lesser in height than the existing principal dwelling.

Size is where we have to be careful, because the county's own materials do not agree with each other. The Accessory Dwelling Units page states the ADU is limited to a maximum of 75% of the gross square footage of the principal dwelling or 1,000 square feet, whichever is less. The county's Development Services FAQ, answering the same question, says 50% of the livable square footage or 1,000 square feet, whichever is less.

Those produce very different answers on a 1,200 square foot house — 900 square feet against 600. We are not going to pick one for you. Confirm the current figure with Planning in writing before you design anything, and see the caveats below.

Gross square footage, for these purposes, is the interior habitable area — the county glosses it as the area that is heated or cooled.

What cannot be an ADU, and the one that can

Recreational vehicles, railroad cars and camper shells are not permitted for use as an ADU in any zoning district. That is worth reading next to the separate rule that you may live in an RV during construction with a temporary use permit — a permitted temporary occupancy and a permanent second dwelling are different things, and the first does not become the second by the passage of time. Our post on living in an RV while you build covers that side.

Park model trailers are the exception, and they are a statutory one. Under A.R.S. § 11-820.03 the county must allow a park model trailer as an ADU for use as a single-family residence on any parcel zoned for one dwelling unit per three acres or greater, placed on a semi-permanent or permanent foundation and connected to utilities.

The installation standards are specific:

Requirement	Standard
Size	Gross interior area at least 320 and no more than 400 square feet
Foundation	Concrete pads and piers with Minuteman anchors, or concrete pads and piers with block skirting on a footing using straps and wedges
Skirting	Vinyl skirting if block skirting is not used
Utilities	Proper connections — water, septic, electric
Door landings	36 by 36 inches at all door openings; a handrail where there are four or more risers
Axles and wheels	All must be removed

That last line is the philosophical one. The park model stops being a vehicle and becomes a building, and the county wants that to be irreversible.

Manufactured homes and rehabilitated mobile homes may be permitted as an ADU only in zoning districts where they are already a permitted use.

The practical constraints people hit

Ownership cannot be split. An ADU may not be subdivided or otherwise segregated in ownership from the principal dwelling. If the plan was to build a second house and sell it separately, that is a land division question, not an ADU one.

Septic has to carry it. If the ADU uses a septic system, the system must be sized to handle the additional waste, and enlargement or an entirely new system may be required. This is frequently the real cost of an ADU and it is worth pricing before the walls — our septic guide covers the evaluation and the reserve area, which the ADU also cannot sit on.

Utility easements are hard boundaries. An ADU may not be built on top of a current or planned public utility easement unless you have written consent from any utility currently using it or that may use it in future. Note "may use in future" — the absence of a line today is not the absence of an easement.

Utilities can be shared or separate. Meters may be either, which matters more on an off-grid parcel than a metered one, because a second dwelling is a second load through a December week. See off-grid solar in winter before you assume the existing array will carry it.

When you need more than administrative approval

• A Special Use Authorization is required to use an ADU for commercial purposes, or for any use other than a permitted home occupation.
• A Variance is required for any deviation from the development standards.

The process itself is straightforward: application plus a site plan showing compliance, submitted through the county's permitting portal, reviewed by planning staff, decided in writing by email as approval, denial or conditional approval. Only then do you apply for the residential building permit.

What we could not confirm

• The maximum size. The county's ADU page says 75% of gross square footage; its Development Services FAQ says 50% of livable square footage. Both cap at 1,000 square feet. We could not determine which is current and we are not going to guess — get it in writing from Planning before you design.
• We could not confirm whether "livable" and "gross" square footage are intended to mean the same thing in those two sources, which may be the whole explanation or may not be.
• We could not confirm current application or permit processing fees. The county's planning fee schedule is published in its ordinances.
• We could not confirm how the owner-occupancy requirement is monitored or enforced over time, or what happens if the owner later moves away.
• We could not confirm whether a park model that meets the 320 to 400 square foot range is also subject to the percentage size cap, or whether the statutory allowance overrides it.
• We could not confirm which zoning districts in the valley permit manufactured homes, which decides whether that route is open on your parcel.
• This is a summary of county materials, not planning advice for your parcel. Development Services is the authority.

The short version

A second dwelling on one parcel is allowed, but as a planning application that must be approved in writing before any building permit is issued. You need an existing single-family home first, you must live on the parcel yourself, and the ADU must be no taller than the main house. The county permits at least one attached and one detached ADU per parcel, plus one additional detached ADU on lots of an acre or more. An RV, a railroad car or a camper shell can never be the ADU; a park model can, under A.R.S. § 11-820.03 on parcels zoned one dwelling per three acres or more, at 320 to 400 square feet, on a proper foundation, with the axles and wheels removed. Ownership can never be split from the main house. And check the size cap directly with Planning, because the county's own two published answers do not match.

Sulfur Springs Land Management does the pads, access and utility trenching a second dwelling needs. The septic capacity question is usually the one worth answering first.