A rough order of magnitude (ROM) estimate is an early cost range used to judge whether a project is worth pursuing before any meaningful design work begins.
A ROM answers one question: is this project in the right financial neighborhood? It is built from historical costs, unit rates, and comparable projects rather than quantities taken from drawings, and it is only as good as the assumptions written next to it.
Where ROM sits among estimates
Estimates tighten as design advances. AACE International’s estimate classification system places the ROM at Class 5, often called concept screening, prepared with roughly 0 to 2 percent of the project defined. AACE’s expected accuracy for a Class 5 estimate is wide: from 20 to 50 percent below the final cost to 30 to 100 percent above it. Budget estimates (Classes 4 and 3) narrow that range as programming and schematic design firm up, and definitive estimates (Classes 2 and 1) are built from detailed drawings and quantities.
A parking example
WGI’s 2026 Parking Structure Cost Outlook puts the national median construction cost of a parking structure at $33,300 per space, or $98.75 per square foot, up 6 percent from the prior year. A ROM for a 500-space above-grade garage starts near $16.7 million. Applying the full Class 5 range, the eventual cost could reasonably land anywhere from about $8.3 million to $33.3 million. That spread is the point: a ROM screens a project in or out, and it should never be dropped into a pro forma as if it were a bid.
The WGI figure is a national median for construction. Costs vary sharply by market (WGI’s range runs from Houston at the low end to San Francisco at the high end), below-grade parking costs considerably more, and land, design fees, financing, and equipment such as PARCS and EV charging sit on top.
Common ROM mistakes
- Presenting a single number instead of a range.
- Leaving out the exclusions, so readers assume land, soft costs, or technology are included.
- Forgetting escalation. At 6 percent a year, a ROM prepared two years before construction is already stale.
- Carrying the ROM forward unchanged after the program changes.
Related terms: Gross Floor Area (GFA), Design-Build
Further reading: The Evolution of U.S. Parking Structures in 2026