Three plumbing bids come in for your 12-unit multifamily project. The numbers: $88,000, $104,000, and $127,000. The low bidder is 30% below the others. Before you call them back to celebrate, you need to understand what's actually in each bid — because plumbing is one of the trades where "same scope" rarely means the same thing.
Plumbing accounts for 8-12% of total construction costs on residential and light commercial projects, and the variance between bids is almost never pure labor efficiency. It's scope. One contractor included a $4,200 fixture allowance; another specified real fixtures. One included a gas line connection; another excluded all gas work. One priced hot water recirculation; another didn't know it was in the specs.
Here's what experienced GCs check when they're comparing plumbing bids — and the scope gaps that turn the low bid into the expensive one.
Why Plumbing Bids Are Hard to Compare
Plumbing bids are difficult to compare for a specific structural reason: the work has two distinct phases — rough-in (pipes, vents, drains installed before walls close) and trim-out (fixtures installed after finish work). Not every contractor bids both, and the ones that do don't always separate them clearly.
A bid that says "complete plumbing installation" might mean everything soup to nuts. Or it might mean rough-in only, with trim-out as a separate phase. You won't know until you ask — or until the contractor shows up for trim-out with a price you didn't expect.
Fixture allowances add another layer of complexity. Some contractors include lump-sum fixture allowances — 'fixtures: $6,500 allowance' — which are placeholder numbers, not real costs. Others specify actual fixtures by manufacturer and model. These two approaches can appear identical in price but differ by thousands when actual materials get purchased.
Scope Items to Verify Across All Bids
Rough-In vs. Trim-Out Phase Coverage
Confirm explicitly whether each bid covers rough-in only, trim-out only, or both. On projects where rough-in and trim-out are separated in time (common on multifamily and commercial), some contractors bid phase 1 only and expect a separate scope for phase 2. This needs to be clear in the contract before work begins.
Fixture Specifications and Allowances
When a bid includes a fixture allowance, it means the contractor expects you to select and approve specific fixtures before the allowance is finalized. If your actual selection costs more than the allowance, the difference is a change order. On a 20-unit project, a $200 per-unit allowance underrun multiplies fast.
Compare plumbing bids automatically
Upload plumbing contractor proposals and EstimateHawk extracts fixture counts, normalizes rough-in vs. trim-out scope, and flags missing items — in 30 seconds.
Try it free — no credit card requiredCompare allowance amounts carefully. A bid with a $3,000 fixture allowance is not the same as a bid with a $6,500 allowance, even if the total bid prices look similar. The lower allowance is just a deferred cost.
Gas Line Work
Gas piping is commonly excluded from plumbing bids — some GCs handle it through a separate mechanical contractor, others expect the plumber to include it. If your project has gas appliances, confirm explicitly whether gas piping to those appliances is included in each plumbing bid or excluded.
Water Heater and Hot Water System
Water heater installation — especially commercial tank or tankless systems — can be a significant cost item that gets excluded or assumed differently by each contractor. On multifamily projects, recirculation systems add another layer: some contractors include the pump and piping; others exclude it entirely and wait to be asked.
Scope Gap Alert
Hot water recirculation systems are excluded from more plumbing bids than any other single line item. If your project specs call for recirculation and you don't verify each bid, you will find out the hard way — usually when one contractor's "value engineering" suggestion saves $8,000 on the recirculation line and creates $15,000 in tenant callbacks.
Red Flags in Plumbing Bids
- Lump-sum bids with no line-item breakdown — no way to evaluate what's included
- Fixture allowances that are 40%+ below typical market cost for specified fixtures
- No mention of testing and balancing procedures
- Permit fees excluded — sometimes a $1,500-3,000 cost left to the GC
- "Underground work per plan" with no explicit inclusion of sleeving or gravel bedding
- No mention of insulation on hot water supply lines (required by code in most jurisdictions)
How to Normalize Plumbing Bids for a Fair Comparison
Once you've identified what each contractor includes and excludes, normalize the bids to a common scope. For excluded items, request add-on pricing. For fixture allowances, adjust to a consistent allowance amount so you're comparing the same placeholder across all bids.
After normalization, the low bid and the high bid often end up much closer than the raw numbers suggested. The goal isn't necessarily to pick the lowest normalized bid — it's to understand what you're actually buying before you sign. Fair bid comparison is about scope, not just price.
AI bid comparison tools handle this normalization automatically. Upload all three plumbing PDFs and the tool extracts every line item, flags scope gaps, and builds a normalized comparison. Items present in one bid but missing from another are highlighted with cost estimates — so you know exactly what each contractor's number actually covers before you pick up the phone.