Residential Infrastructure

How we approach it

Sized for the load, designed for the lifecycle.

The first thing that surprises clients new to multifamily is that the infrastructure sizing has nothing to do with the parcel's acreage. The basic minimums are six- and eight-inch water and sewer mains, PVC throughout, fire hydrant spacing at 500 feet, sanitary manholes at 400-foot maximum spacing, storm pipe minimum fifteen inches in most jurisdictions. Those numbers don't change between a single-family neighborhood and a 200-unit apartment complex — but the demand on them does. A 200-unit project draws water like a small subdivision, generates sewer flow like a small subdivision, and produces storm runoff at the rate of a heavily impervious commercial site. The infrastructure has to be designed for the load, not for the lot.

Water service is the most visible piece. Multifamily projects typically run a master meter at the property line with sub-metering at each building or each unit, depending on the management model. Fire flow is the binding constraint — Standard practice requires fire hydrant placement so that every building falls inside a 250-foot radius circle around a hydrant, and the line size has to deliver the flow the fire marshal calls for at the right pressure. On low-pressure city service we sometimes have to design a pressure boost, which adds an electrical room and a maintenance burden. Bailey works the fire flow numbers in feasibility, not in construction documents, because finding out late that the parcel doesn't have the pressure is a project killer.

Sanitary sewer design is the next critical piece. Multifamily generates concentrated peak flows that demand careful slope design — too flat and the line silts; too steep and the velocity scours. Lateral lines should be placed behind units whenever possible, and the main trunk should be extended to allow long lateral runs to end-of-line units rather than extending the trunk further. On tight Treasure Valley sites with shallow city sewer service, we sometimes have to design a lift station to get the sanitary out — and we design it with redundant pumps, an alarm panel, and access for the city or the HOA's maintenance contractor. Avoiding a lift station entirely is always cheaper if the gravity numbers work.

Storm drainage is the most consequential design discipline on a multifamily site. It is the single most important design element for high-density residential, because the impervious cover is high and the available open space is tight. The right move is to make stormwater facilities work double-duty as community amenities — Detention ponds are valid open space and retention ponds can be designed with permanent water surfaces, with premium units bordering both. Bailey designs every multifamily detention facility as a usable feature: a lawn for weekend dog play, a pond with a perimeter path, or a swale that doubles as a planted buffer. The land doesn't get wasted.

Dry utilities — electric, gas, telecom — are the easiest to coordinate and the most often coordinated late. Joint trenching reduces cost and reduces conflicts in the ground, and Bailey coordinates it from feasibility forward. Transformer pads need to be located near each building's electrical room with access for the utility's service truck. Backflow vaults need to be located where the city water department can inspect them without blocking parking. Gas service needs to be routed without crossing storm or sanitary in conflict zones. None of it is dramatic, and all of it has to be drawn before the construction documents leave our office.

Irrigation is the last quiet piece. Multifamily landscape and amenity areas — pools, lawns, planted swales, parking islands — need a dedicated irrigation system, and on most Treasure Valley sites the source is a lateral from the Nampa-Meridian Irrigation District (NMID), a delivery from the Boise Project Board of Control (BPBC), or a city pressure-irrigation hookup. NMID and BPBC easements run through almost every parcel in Ada and Canyon counties, and the multifamily site plan has to accommodate them. Bailey coordinates the irrigation sleeves under driveways and parking before the paving goes down, because cutting them in afterward is expensive and visible.