Contract structure doesn’t just determine how a general contractor gets paid. It determines who absorbs the cost risk between groundbreaking and stabilization, and that allocation flows directly into your return model. On a multifamily deal underwritten to a 150–250 basis point development spread, a construction contract that quietly shifts overrun risk onto the sponsor can erode that spread faster than a soft rent comp or a cap rate move the market didn’t warn you about.
Guaranteed Maximum Price (GMP) and cost-plus are the two delivery structures multifamily developers weigh most often in Los Angeles and Orange County, and the right answer isn’t universal. It depends on how complete your design is at contract signing, how much schedule certainty your capital partners need, and how much you’re willing to pay for a ceiling. This guide breaks down how each structure actually protects or exposes your IRR.
The Core Difference: Who Owns the Overrun
| Cost-Plus | GMP | |
|---|---|---|
| Who absorbs cost overruns | Owner | Contractor, above the ceiling |
| Fee structure | Cost of work plus a fixed or percentage fee (typically 10–20%) | Cost of work plus fee, capped at a guaranteed maximum |
| Contingency ownership | Typically owner-held | Split between GC contingency (2–5%) and owner contingency (5–10%), depending on negotiation |
| Cost transparency | Fully open-book | Open-book, with negotiated definition of what counts against the cap |
| Best fit for design completeness | Early-stage design, fast-track starts | Design substantially complete (typically 80–90%+ documents) |
| Upside on savings | Owner keeps 100% of savings (no ceiling to share against) | Typically shared between owner and GC per a negotiated split |
A GMP contract is, structurally, a cost-plus contract with a cap layered on top, which is why the real underwriting question isn’t “which structure” in the abstract, but how much design certainty you have when you need to sign.
How Each Structure Shifts Risk in a Multifamily Deal
Under cost-plus, the sponsor carries the full exposure to cost growth. If steel, labor, or MEP pricing moves against you mid-construction, that cost flows straight to the project budget, and from there, straight to your yield-on-cost. The upside is real: cost-plus avoids the contingency premium a contractor builds into a GMP to price the risk they’re absorbing, and the owner captures 100% of any savings rather than sharing them. For a sponsor with a strong, trusted GC relationship and a design team that can hold scope, cost-plus can be the cheaper structure on paper.
Under GMP, the contractor absorbs cost growth above the ceiling which is exactly the protection a capital partner or lender underwriting to a fixed development spread wants to see. That protection isn’t free: the GC prices contingency into the GMP to cover the risk they’re taking on, and that contingency is baked into your total project cost whether or not it’s ever used. If the project comes in under the GMP, savings are typically shared, not fully returned to the owner, meaning a well-run GMP project can still leave money on the table that a well-run cost-plus project would have kept.
Ironside structures GMP contingency and shared-savings terms explicitly at the term sheet stage with our multifamily clients, because vague contingency language is where the two structures stop functioning as intended for either party.
Contingency: The Line That Actually Determines Which Structure Protects You
Contingency ownership is where the real IRR protection question lives, and it’s frequently under-negotiated:
- GC contingency (typically 2–5% of construction cost) is held by the contractor to absorb bid errors, productivity shortfalls, and subcontractor performance issues within the defined scope. This exists in both structures but is only capped in a GMP.
- Owner contingency (typically 5–10% of construction cost) funds scope additions, design changes, and unforeseen conditions the GC’s contingency doesn’t cover. This should be explicitly owner-controlled in either structure, and in our experience, is the single most contested line item in GMP negotiations.
- Unused contingency disposition is a contract term, not a default. Some GMP contracts return 100% of unused owner contingency to the sponsor; others split it as shared savings. This single clause can shift several hundred basis points of construction budget back toward or away from your projected returns.
A sponsor who negotiates a GMP without clearly defining contingency ownership and disposition is effectively signing a cost-plus contract with extra paperwork and a false sense of ceiling protection.
How a Cost Overrun Flows Through to IRR: An Illustrative Example
Consider a multifamily development underwritten to a 7.5% yield-on-cost against a 5.0% exit cap rate. A 250 basis point development spread, comfortably inside the range sponsors typically target to absorb normal execution risk. If construction costs run 10% over budget on a $30M hard-cost project, that’s a $3M erosion of project cost basis with no corresponding increase in stabilized NOI. Depending on capital structure and timing, that kind of overrun can compress the development spread by well over 100 basis points. Turning a deal with real margin of safety into one trading close to the market cap rate with little cushion left for lease-up softness or a rate move before permanent financing.
Under a cost-plus structure, that entire $3M lands on the sponsor’s balance sheet. Under a well-negotiated GMP, a meaningful share of that overrun. Everything above the ceiling, sits with the contractor instead, protecting the spread your capital partners underwrote to. This is the actual mechanism by which contract structure protects IRR: not the paperwork, but where the marginal dollar of cost growth lands when a project doesn’t go exactly to plan.
This example is illustrative for underwriting discussion, not a specific project projection. Run the actual sensitivity against your own pro forma before treating any spread figure as a target.
When Each Structure Fits Best in LA and OC Multifamily Deals
- GMP tends to fit better when design is substantially complete before you need to break ground, when a lender or LP requires a firm cost ceiling as a financing condition, and on entitlement-heavy sites in Los Angeles or Orange County where schedule risk from plan check and inspection timelines needs to sit somewhere other than your open-ended budget.
- Cost-plus tends to fit better on fast-track projects where construction needs to start before design is fully resolved, on deals with a long-standing, high-trust GC relationship, and where the sponsor has the balance sheet and risk appetite to self-insure against cost growth in exchange for capturing full savings upside.
- A hybrid approach. Cost-plus through early phases with a GMP locked in once design reaches a defined completeness threshold, is increasingly common on multifamily projects that need to start site work before permitting fully clears, and lets a sponsor delay the ceiling decision until the estimate underneath it is more reliable.
Ironside works with developers to make this call at the pro forma stage, not after a GC has already been selected, because the contract structure decision and the site/design timeline decision are really the same decision viewed from two directions.
The Risk Factor Underneath Both Structures: Estimate Reliability
Neither GMP nor cost-plus protects an IRR built on an unreliable initial estimate. A GMP set against 60% design documents carries wide contingency because the GC is pricing real uncertainty; a cost-plus budget built the same way just moves that same uncertainty onto the sponsor’s books instead of the contractor’s. The structure determines who holds the risk: proactive, detailed preconstruction estimating is what determines how big that risk actually is in the first place. This is why Ironside pushes for earlier involvement on multifamily deals: a GMP or cost-plus number set against 90% documents and a real subcontractor buyout is a materially different risk than the same structure set against a schematic design.
Build Your Vision with Ironside
Ironside Building is a full-service Southern California general contractor based in Irvine, working with multifamily developers and investors across Los Angeles and Orange County from site evaluation through stabilization. The contract structure decision is easiest to get right when Ironside is involved during underwriting and design development, not after the GMP or cost-plus terms are already on the table.
If you’re evaluating a multifamily site or refining your pro forma, share your current design completeness and timeline with our team, and we’ll walk through which delivery structure protects your specific deal.