Where General Contractors and Builders Lose Margin - and How to Protect It

General contracting sits on a knife's edge. Net margins between 4% and 6% on commercial projects leave almost no room for cost events that go unrecorded, costs that arrive after the client invoice has gone out, or receivables that are never formally chased. Yet the leaks that reduce a $350,000 commercial project from 5% net to breaking even rarely surface mid-project. They appear in the final account - or they disappear entirely into a pattern of good revenue but thin margins. Four specific failure points repeat across general contractors and builders of every size. Each is preventable.

Preliminaries Priced as a Percentage Instead of Built Up Line by Line

Preliminaries - the site management staff, temporary facilities, plant hire, utilities, and compliance costs that keep a project operational without going into the finished structure - typically represent 8-12% of a commercial new-build contract value and 12-20% on refurbishments of occupied buildings (Plexapro, 2025). That range exists because prelims are highly sensitive to project duration, site constraints, and the split between time-related and fixed costs.

Most SMB general contractors price prelims as a flat percentage, often 7-8%, arrived at from experience rather than analysis. On a project that runs to programme, the gap stays manageable. On any project that overruns, the time-related component - site manager salaries, scaffold hire, site office costs, equipment standing charges - continues to accrue regardless of the lump-sum budget. A 2024 industry study of 50 failed or severely delayed projects found that inadequate preliminaries were a contributing factor in 68% of cases, with an average financial impact of 12% of total project value (Plexapro, 2025).

The fix is building prelims line by line, with time-related items stated as a weekly rate. When a project extends through client-side delays or programme revisions, a declared weekly rate is the basis for a recoverable claim under standard JCT and NEC contract terms. A lump sum has no equivalent protection. Contractors who protect margin here split every prelims item into fixed (mobilization, site establishment, demobilization) and time-related (site management, equipment hire, ongoing running costs), then price the time-related component per week. A prelims register maintained from mobilization to practical completion tracks actual cost against budget in real time.

Client-Directed Changes Absorbed Without Written Variation Orders

The most expensive thing that happens on a construction site is a client making a decision informally. A call from the contract administrator asking to relocate three partition walls. A request to add power outlets to a room that was out of scope. A design change that extends groundworks by two weeks. In each case, a cost event occurs before a formal record of it exists.

That gap between the instruction on site and the written variation order is where general contractor margin disappears. Analysis of change event management consistently identifies the recognition lag - the days between a change occurring and it entering a formal commercial record - as the primary driver of unrecovered entitlement (Xpedeon, April 2026). Every day in that gap, evidence of who instructed the work and what it cost weakens. Under NEC and JCT contracts, the notification window is specific. Late submission does not just create an administrative issue; it creates a recovery risk.

For a general contractor running ten active projects with three to four client-directed changes per project at an average value of $1,500-$3,000 each, the difference between capturing every change in writing and absorbing half verbally runs to $45,000-$120,000 per year in unrecovered entitlement. The fix is a site instruction log from day one, a written variation threshold of $150-$200 above which written client approval is required before proceeding, and a rule that no additional work starts without a signed instruction in hand or a written notification sent within 24 hours.

Sub-Contractor Costs Landing After the Client Invoice Has Gone Out

General contractors who sub-contract specialist trades face a sequencing problem that erodes final account margin. The client invoice goes out on practical completion. Sub-contractor invoices arrive the following week. When those invoices include additional scope agreed verbally on site, or quantities that exceed the purchase order by 10-15%, the cost lands after the revenue has been recognized and the billing opportunity has closed.

Sub-contractors typically represent 15-25% of total project cost on a commercial job (Siana, 2026). On a $600,000 project with $120,000 in sub-contracted packages, invoices arriving 10-15% above the agreed PO value represent $12,000-$18,000 in cost that cannot be passed through to the client. At 5% net margin, the project target was $30,000. A single sub-contractor cost surprise eliminates one-third to one-half of it.

Two disciplines close this gap. Every sub-contractor booking requires a written confirmation with a fixed price, a clearly defined scope, and a purchase order before mobilization - verbal additions on site require a written day work sheet approved before the sub-contractor leaves. Client invoices should not go out until all sub-contractor invoices for that project phase have arrived and been matched against their POs. Requiring sub-contractors to invoice within five working days of completing their scope keeps the hold short. A three-way match on each package - PO, completion sign-off, sub-contractor invoice - before payment also catches overcharges before they become disputes.

Retention Not Tracked as a Named Receivable with Explicit Release Dates

On commercial projects, clients typically withhold 5% of each payment as retention. The structure is straightforward: 50% released at practical completion when the practical completion certificate is issued, and the remaining 50% held through the defects liability period (typically 12 months) and released when the final certificate is issued.

For a general contractor running 10-15 commercial projects per year at average contract values of $300,000-$500,000, retention receivable on the books at any time runs to $90,000-$187,500 at 5% across the portfolio. Many of those projects will have reached practical completion months ago. The DLP may have ended. But if the retention invoice was never raised and never chased, the money stays with the client by default. Mobilization Funding's 2025 Construction Delays and Payment Timing Report found that most contractors wait 60 days or more for payment even on 30-day terms - and that pattern is more pronounced on retention sums where the trigger date is unclear to both parties.

The fix is treating retention as a named receivable from contract award. Log the retention percentage, the total sum, the practical completion date, and the DLP end date at contract stage. Raise the 50% retention invoice the same day the practical completion certificate arrives. Set a calendar reminder 30 days before DLP end to chase any outstanding defects. Issue the final retention invoice on DLP end without waiting for the client to ask.

General contractors who consistently protect margin do four things: they build prelims line by line with weekly rates on every time-related item, they capture every client instruction in writing before work proceeds, they hold client invoices until sub-contractor costs are reconciled, and they manage retention as a tracked cash receivable with explicit release dates. None of it requires a complex system - it requires making each discipline a non-negotiable part of how every project runs.

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