What is floor area ratio (FAR) and which Arizona cities use it?

No matter how the stories are arranged, one rule limits how much house you can build on a lot: floor area ratio (FAR), which caps the total floor area of a home as a multiple of the lot size. The formula is simple: FAR equals floor area divided by lot area. The American Planning Association states it plainly: a FAR of 1.0 means the floor area may equal the lot area, and a FAR of 0.5 means the floor area may be no more than half the lot area. So a 0.25 FAR on a 40,000-square-foot lot allows up to 10,000 square feet of total floor area, counting all stories together. In Arizona, FAR is used most clearly by Paradise Valley and Tucson, and it appears in parts of the Phoenix zoning ordinance, while many smaller metro towns control size with lot coverage and height instead.

FAR matters because it can be the rule that decides your home's size, more than setbacks or height. A two-story design does not escape it, since FAR counts every floor. This page explains the formula, walks a worked example, names the Arizona cities that apply it, and shows how it works with the other zoning limits. For the full set of rules that size a home, see what is the biggest house I can build on my lot in Arizona.

How floor area ratio works

Floor area ratio sets a hard ceiling on total floor area by tying it to the lot size, so a bigger lot earns a bigger house and a small lot does not. The American Planning Association describes FAR as a single ratio assigned to a zone that allows variable building dimensions within an overall floor-area limit. It controls bulk, not shape. You can spread the allowed floor area across one story or two, but the total cannot exceed the cap.

The math is one step. Multiply the lot area by the FAR to get your maximum floor area:

• Lot area times FAR equals maximum floor area. A 0.30 FAR on a 12,000-square-foot lot allows 3,600 square feet of total floor area.
• FAR counts all stories. Unlike lot coverage, which only measures the ground footprint, FAR adds up every floor. A two-story home uses the same FAR budget as a single-story home of the same total size.
• What counts can vary. Most ordinances count enclosed, conditioned floor area. Garages, covered patios, and unenclosed space may or may not count, depending on the city's definition. Always check how your city defines "gross floor area."

This is the key difference from lot coverage, which caps only the share of the lot the building footprint covers on the ground. A lot can pass coverage and still be capped by FAR, because a second story adds floor area without adding footprint. See our lot coverage glossary entry for how the two rules differ and work together.

A worked example on an Arizona lot

On a one-acre Paradise Valley lot with a 0.25 FAR, you could build up to about 10,890 square feet of total floor area before other limits apply. Here is the math, using FAR the way Arizona cities apply it.

Take a one-acre lot, which is 43,560 square feet, in a district with a 0.25 FAR:

• Lot area: 43,560 square feet.
• FAR: 0.25.
• Maximum floor area: 43,560 times 0.25 equals 10,890 square feet, counting both stories.

That number is the ceiling on living area, garage, and any other counted space combined. If you wanted a single-story home, the footprint would be limited by setbacks and lot coverage before it reached the FAR cap. If you went two stories, FAR is often the rule that runs out first, because the second floor adds area without adding footprint. The binding limit is whichever rule caps you first. On large-lot estate districts like Paradise Valley's, FAR commonly controls the answer, which is why owners there design to the FAR from the start.

Smaller FAR numbers protect a low-density, open feel. A 0.25 FAR on an acre keeps homes well under a quarter of the lot in floor area, which is how an estate community holds its character. A higher FAR allows a denser build. The point is the same: pull your district's FAR before you design, because it sets the size ceiling.

Which Arizona cities use FAR

In Arizona, Paradise Valley and Tucson are the clearest residential FAR users, and Phoenix applies FAR in parts of its ordinance, while many metro towns rely on lot coverage and height instead. FAR is not universal in the state, so whether it applies to your lot depends on the city and district.

Where FAR shows up:

• Paradise Valley. The town uses FAR to cap single-family floor area in its low-density estate districts, with a FAR commonly set around 0.25 for its larger-lot zones. FAR is a central control there because it preserves the open, low-density look the town is known for.
• Tucson. Tucson's Unified Development Code applies FAR to set the maximum floor area allowed on a site, calculated by multiplying the lot area by the zone's FAR. It uses FAR alongside setbacks, height, and lot coverage.
• Phoenix. The Phoenix zoning ordinance defines and uses FAR in parts of its code, though many single-family R1 districts size homes mainly through lot coverage and height rather than a residential FAR.
• Many smaller metro towns. Scottsdale, Cave Creek, Fountain Hills, and unincorporated Maricopa County more often control home size with lot coverage and height limits, not a residential FAR. Always confirm your specific district.

Because FAR use varies, the safe move is to pull your lot's district standards and look for a FAR line. If the district lists one, it is likely the rule that sets your size ceiling. If it does not, lot coverage and height will. For how those combine, see what is the biggest house I can build on my lot in Arizona.

How FAR fits with setbacks, height, and coverage

FAR works together with setbacks, height, and lot coverage, and the largest home your lot allows is whatever fits inside all of them at once. No single rule gives the full answer. Each one limits the home in a different way, and they are all checked at plan review.

How the four limits interact:

• Setbacks push the home in from each property line and set the flat area it can sit on, the buildable area. See our setback glossary entry.
• Height caps how tall the home can be, which controls how many stories you can stack. Most Arizona single-family zones run 24 to 30 feet. See how tall can a custom home be in Arizona.
• Lot coverage caps the ground footprint as a share of the lot.
• FAR caps the total floor area across all stories as a multiple of the lot.

The binding rule is whichever one caps you first. On a small lot, setbacks and coverage often run out before FAR. On a large estate lot, FAR is frequently the tightest limit, since the lot is big enough that coverage and setbacks leave plenty of footprint but the FAR holds down total floor area. Stack all four on your lot and the space they leave is your buildable envelope, the three-dimensional room your home has to fit inside.

Zoning standards change, and not every city applies FAR, so confirm your lot's current district, FAR (if any), setbacks, height, and coverage with your city or county planning department before you design. The cleanest path is to have your builder or designer pull every applicable limit and draw the buildable envelope on the first site plan, so you know your true size ceiling before you commit to a floor plan. For the complete walkthrough, see what is the biggest house I can build on my lot in Arizona.

Where Jematell Homes comes in

Vetting whether a parcel is truly buildable is where we start every project. If you are planning a custom home in Scottsdale, Rio Verde, or the greater Phoenix metro, we are happy to walk through your project.

Sources

• American Planning Association (PAS Report): Floor Area Ratio definition and formula
• City of Tucson: Zoning Code (Unified Development Code, residential FAR)
• Town of Paradise Valley: Zoning Ordinance (single-family floor area ratio)