Multifamily GMP cost risk, and the three lines that move most

Multifamily GMP cost risk is unusually concentrated. A residential building is hundreds of repeated units, so most of the price is predictable. Three lines are not, and they account for most of the movement on almost every job.

Why repetition makes the rest of the price reliable

A two hundred unit building is not two hundred different problems. It is one unit built two hundred times, plus a structure, plus an envelope, plus the parts that are not repeated.

That repetition is why multifamily estimates are usually good. A unit type priced once and multiplied is a reliable number, the trades are familiar, the details are standard, and the learning curve on site works in everybody favor by the third floor.

So the parts of the price that cover units are rarely where the trouble is. Drywall, doors, cabinets, plumbing fixtures and unit finishes behave.

What does not repeat is the first floor, the envelope and the site. Those three are bespoke on every building, they carry most of the interfaces, and they hold most of the allowances.

This is a useful thing to know before signature, because it tells you where to spend a limited amount of reading time. A page by page review of the unit finish schedule is a poor use of an afternoon. A page by page review of the qualifications page is not.

The rest of this note is the three lines and what makes each of them move.

The three lines where multifamily GMP cost risk actually sits

One. Site work and below grade. Almost every uncapped exclusion on a residential job lives here: unsuitable soils, rock, dewatering, existing utilities, contaminated material and utility company connection charges. The work is unpriceable without a geotechnical report and a utility survey, so it is carried as an allowance or excluded outright.

Two. The envelope. Window and door packages, cladding, balconies and every junction between them. Balconies are the specific item: they penetrate the wall assembly, they carry structure, drainage and railing, and the detail at that junction is drawn once and built several hundred times. A change there multiplies.

Three. The podium or first floor. Whatever sits under the residential floors, whether that is parking, retail, amenity or a transfer structure. It is bespoke, it holds the mechanical and electrical service entries, and it is where the building meets the site.

Those three typically represent a third of the contract value and rather more than half of the realistic open exposure.

The rest of the building, which is most of it by value, is the part everybody spends the meeting discussing.

What makes the envelope line move

Two mechanisms, and they are different from each other.

The first is multiplication. A window detail that costs two hundred dollars more than the price carried is eight hundred windows on a mid size building, which is $160,000 from one detail nobody looked at. The same arithmetic applies to balcony thresholds, railing connections and the transition at every floor line.

The second is performance. Residential envelopes carry air and water performance requirements, and the drawn assembly frequently does not achieve the specified performance without an upgrade. That conflict is a drawings against specification problem, and it usually surfaces at the mockup, which is after the package is awarded and after the units are ordered.

The useful test before signature is to take the three most repeated envelope details and ask what they were priced from: a drawing, a performance requirement, or a previous project. The answers differ more often than owners expect.

The same reconciliation, in general terms, is set out in what a guaranteed maximum price actually guarantees.

A worked example

A 218 unit building at a $63 million guaranteed maximum price, with 4 percent contingency, so $2.52 million.

Unit interiors, corridors and the repeated portion of the building: $41 million, priced from a unit type multiplied out, with two small allowances.

The three lines: site work and below grade $6.1 million with three uncapped exclusions, envelope $9.4 million with the balcony detail carried against a preliminary section, podium $6.5 million with four allowances.

Realistic open exposure across the three: $1.8 million to $3.4 million. Across everything else: roughly $180,000 to $320,000.

So 35 percent of the value carries about 90 percent of the exposure, against a contingency spread evenly across the whole contract.

Nothing is wrong with the estimate. The concentration is a property of the building type, and it is the reason a general percentage protects a multifamily job less well than it appears to.

What the unit count does to everything else

Repetition is an advantage on price and a liability on error.

A mistake inside a unit is multiplied by the unit count before anybody notices it, and on a residential building the first three floors are built before the first inspection of a completed unit happens. Anything wrong in that unit is wrong in sixty others.

This is why the mockup unit matters more on multifamily than on any other building type, and why it is worth insisting that it is built early, built completely and inspected by people who will later have to live with the answer.

The same arithmetic applies to owner decisions. A finish selection changed after the first forty units are complete is a change order on forty units plus the remaining order, and the same decision made two months earlier costs the difference in material and nothing else.

Neither of those is a contract problem. Both are calendar problems, and the calendar is visible at signature.

There is a third effect that shows up in the schedule rather than in the price. Residential floors are built on a cycle, typically five to seven working days per floor, and that cycle is the heartbeat of the whole job. Anything that interrupts it, a late material delivery, a failed inspection, a design clarification, costs a full cycle rather than the hours actually lost.

So on a multifamily building the schedule risk is also concentrated, in the sense that a small number of interruptions to the floor cycle account for most of the delay. The items capable of interrupting it are worth listing at signature: the envelope material, the elevator, the electrical gear and any inspection the jurisdiction performs floor by floor.

What to do before you sign

1. Split the price into repeated and non repeated work, and read the non repeated part first.
2. Count the uncapped exclusions in site work and below grade, and price each one.
3. Confirm a geotechnical report exists and that the price was built from it rather than around it.
4. Take the three most repeated envelope details and establish what each was priced from.
5. Check the balcony detail specifically, including drainage, railing connection and thermal break.
6. Require the mockup unit early in the schedule, complete, and written into the contract.
7. List every owner finish decision with the date it has to be made before it multiplies.

Item one reorders the whole exercise and takes ten minutes. Most owners read a multifamily estimate front to back, which means the bulk of the attention lands on the bulk of the value rather than on the bulk of the risk.

Item three is the one that decides how the first six months go. A residential job priced without a geotechnical report has an unbounded line in it, and the cost of commissioning the report is a rounding error against what that line can become.

The scope options for this reading sit in the review packages.

What we do

We split the price into the repeated and the bespoke, then read the bespoke part in detail: site exclusions, envelope details against the specification, and the podium interfaces. The output is a register ranked by what each item is worth if it moves, with the page reference behind it. It is a commercial reading rather than a design review. The work is the readiness review.

Questions people ask