Does the ADA cluster rule apply to a single porta potty?

The ADA cluster rule does not apply to a single portable toilet at a private backyard party. That rule, from the 2010 ADA Standards for Accessible Design, requires that when multiple single-user portable toilets are clustered at one location, at least 5 percent of the units (practically, at least one) must be an ADA-compliant unit. A cluster means two or more units. Since you have only one standard unit, the rule is not triggered by the ADA.

For your event with 15 guests, one standard unit fits the common units per person guideline, which starts around one unit per 50 to 75 people for a four-hour event. With alcohol served, having just one unit is acceptable for that group size and duration.

However, to ensure your friend who uses a wheelchair can use the restroom comfortably, you should switch to an ADA-compliant unit. An ADA-compliant unit is larger, has a wider door, grab bars, and a turning space for a wheelchair. The rental company can substitute it for your standard unit. The cost may be slightly higher due to the larger size and features, but for a one-day event the difference is usually small. Confirm with the company that the unit meets the current ADA specifications for accessible design.