You are looking at two lots the same size, three blocks apart. On one, your architect says six units. On the other, they say the number depends entirely on the height map, the rear yard and how much light each unit gets — and they will not guess until they have drawn the box. The second lot is in an RTO district, and the difference is not caution. It is that the usual density formula does not exist there.
The short answer: RTO stands for Residential Transit Oriented. These are moderate-density districts within roughly a quarter mile of frequent transit, and their defining rule is that dwelling unit density is not controlled by lot area — it is controlled by height, bulk, setbacks, open space and unit exposure (§ 207.6). In exchange, projects of five or more units must include family-sized homes.
Key numbers
| Control | RTO | Source |
|---|---|---|
| Dwelling unit density | Not set by lot area — set by the building envelope | § 207.6 |
| Unit mix requirement triggers at | 5 or more dwelling units | § 207.6 |
| Front setback | Average of adjacent properties, never more than 15 feet | § 209.4 |
| Front setback landscaping | 50% permeable, 20% unpaved and planted | § 209.4 |
| Rear yard | 30% of lot depth, never less than 15 feet | § 209.4 |
| Side yard | Not required | § 209.4 |
| Corner commercial depth from the corner | 50 feet maximum | § 231 |
| Narrow street definition | Right of way 40 feet wide or less | § 261.1 |
What these districts actually are
The code describes RTO and RTO-M — the Mission variant — as multi-family moderate-density areas, primarily areas formerly designated RM and RH-3, that sit within short walking distance of transit and neighborhood commercial areas. It puts a number on "short": generally less than a quarter mile. The transit on nearby streets is frequent, or provides multiple lines serving different parts of the city and region.
The physical description matches what you see walking those blocks. A fine-grain pattern of 25-foot to 35-foot building widths prevails, and structures typically run two to five stories. One- and two-family buildings are present, but the district's character is primarily buildings with three or more units in a range of sizes. Buildings are moderately scaled and segmented, and units or groups of units have separate entrances directly from the street.
The code is also explicit about cars: because transit is close and shops are within walking distance, many households do not own one, it is common that not every dwelling unit has a parking space, and overall off-street residential parking is limited. Open space comes from rear yards, decks, balconies, roof decks and courtyards, supplemented by nearby parks and plazas.
Density by envelope, not by arithmetic
This is the rule worth understanding before anything else:
"Dwelling unit density is not controlled by lot area in RTO... but rather by the physical constraints of this Code." (§ 207.6)
The named constraints are height, bulk, setbacks, open space and dwelling unit exposure. The stated purpose is to foster flexible and creative infill development while maintaining the district's character.
What this changes in practice is the order of the work. In a lot-area district you can do arithmetic first and design second. Here you cannot: the unit count is an output of the design, so the envelope has to be established — height and bulk limits for the parcel, the rear yard, the front setback, how each unit will meet the exposure requirement — before anyone can honestly tell you how many homes fit.
It also means two identically sized lots can yield different numbers, because the height and bulk map, the block's setback pattern and the shape of the lot are not identical.
The trade: family-sized units
Removing the lot-area cap came with a condition. To ensure an adequate supply of family-sized units, new residential construction must include a minimum percentage of units with at least two bedrooms.
The requirement applies to all applications for building permits and Planning Commission entitlements proposing the creation of five or more dwelling units. It does not apply to buildings where 100 percent of the residential use is group housing, below-market-rate units provided under the code's inclusionary provisions, single room occupancy units, or student housing.
So a four-unit project is outside it. A five-unit project is inside it, and the mix requirement shapes the floor plans from the first sketch.
Setbacks, landscaping and the street
The front setback rule differs from the older residential districts in a way that is easy to skim past. In RTO it is based on the average of adjacent properties, capped at 15 feet where it is derived that way. The house-and-mixed districts derive theirs from the shortest adjacent setback and cap it at 10 feet. Same idea, different arithmetic, different result on a block with uneven setbacks — check which district you are in before assuming.
The front setback also has to perform. At least 50 percent of it must be permeable so stormwater can infiltrate, and 20 percent must be unpaved and devoted to plant material. Street trees and streetscape improvements are required.
The rear yard is 30 percent of lot depth, never less than 15 feet. Side yards are not required. New buildings are subject to the Residential Design Guidelines.
Street frontage controls apply to residential frontages, with additional controls where limited commercial uses are present. Curb cuts are restricted on certain specified streets and on transit preferential streets, neighborhood commercial streets, and official city bicycle routes and lanes — a real constraint if your plan depends on a driveway.
Corner stores are permitted, within a box
RTO blocks are otherwise residential, but small neighborhood retail is allowed on corner lots only, to put goods and services within walking distance. The rules are tight and specific (§ 231):
- Location. Completely within the district, on or below the ground floor, on a corner lot, with no part of the use extending more than 50 feet in depth from the corner. The deeper 100-foot allowance applies to the higher-density mixed districts and to RTO-M, not to RTO.
- Not at the cost of housing. The space may not require the residential conversion of a residential unit or unauthorized unit — unless the space being converted is a garage or storage space in the basement or first story.
- Which uses. Any use permitted for the first story and below of a neighborhood commercial district or special use district within a quarter mile. If nothing is that close, the neighborhood commercial cluster district's list applies.
- No parking. Accessory off-street parking is prohibited for these uses, deliberately, to keep them walk-in businesses.
The district description adds that only retail compatible with housing is permitted, auto-oriented uses are not, and hours of operation are restricted.
Alleys have their own ceiling
Height in RTO varies by the height and bulk map, but one overlay applies citywide and hits these districts hard. A narrow street is a public right of way 40 feet wide or less, or a mid-block passage or alley under that width created under the code's own passage requirements.
In RTO — along with the two- and three-family house districts, the mixed districts, and the neighborhood commercial districts — a subject frontage is any building frontage abutting a narrow street that is more than 60 feet from an intersection with a street wider than 40 feet. Heights along those frontages are limited to keep narrow streets from being overshadowed and overcrowded, and to preserve sunlight and air. The code also treats east-west narrow streets — those oriented within 45 degrees of true east-west — as their own category.
If your lot fronts an alley, the height map alone will overstate what you can build.
What to check next
The district tells you the framework; the parcel tells you the number. Before pricing a project, confirm the height and bulk designation for your specific lot, whether any frontage is a narrow street, the adjacent properties' front setbacks, and whether you cross the five-unit line that triggers the mix requirement.
And note that being in a transit-rich district often makes state housing laws relevant on top of local zoning, which can change the calculation again. The San Francisco jurisdiction page is a reasonable place to start on the surrounding rules.
Sources
- San Francisco Planning Code § 209.4 — RTO, Residential Transit Oriented Districts
- San Francisco Planning Code § 207.6 — Required minimum dwelling unit mix
- San Francisco Planning Code § 231 — Limited corner commercial uses in RH, RTO and RM districts
- San Francisco Planning Code § 261.1 — Additional height limits for narrow streets and alleys
