Builder Contract Types Explained: Fixed Price, Cost-Plus & GMP

Construction projects can be delivered in several ways, and the type of builder contract you choose affects your cost, timeline, and quality of craftsmanship. Whether you're building a custom home from scratch or managing a renovation, understanding the difference between fixed price, cost-plus, and GMP builder contracts can protect your budget — and your relationship with your contractor.

This guide breaks down each contract type, when to use it, and how to choose the right one for your custom home project.

Design–Bid–Build is the most traditional residential construction delivery method. It works best when you have a complete, detailed set of construction drawings before a single bid goes out. 

Here's how it works: your architect or designer finalizes the full construction documents, then you solicit competitive bids from at least three general contractors and key trades. The result is a lump sum contract — also called a fixed price contract — where the contractor agrees to complete the entire scope for one set number. The goal is to compare bids accurately and select the best value for a clearly defined scope of work. 

Our house plan construction drawings are designed with this method in mind — they include detailed drawings that give contractors what they need to bid accurately, saving time and reducing ambiguity from day one.

Ready to take this conversation to your builder? Check out our Study Set — a complete package of house plan construction drawings your contractor can use to give you a lump sum price before you break ground.

With this method, you bring a contractor into the project early — often during the design phase — rather than waiting for a complete set of drawings. This is common on high-quality or complex custom home projects where craftsmanship and speed both matter. 

Because the contractor is involved from the start, construction can begin on early phases (like site work or foundation) while the design team continues refining interior details. This overlap saves time without sacrificing quality.

CM/GC projects typically use one of two cost structures:

In a cost-plus builder contract, the contractor bills you for all actual costs — labor, materials, subcontractors — plus an agreed-upon contractor markup percentage for overhead and profit. That markup typically runs 15–25% depending on project size and complexity, though it varies by market and contractor.

Everything is transparent and backed by receipts or supplier quotes. This promotes trust and allows for design flexibility, but the final budget can be unpredictable.

Real example: cost-plus in practice
A colleague working on a high-end glass pavilion on the East Coast started with a $4M cost estimate that grew to $10M by completion. Cost-plus contracts can expand significantly when scope evolves or finish levels are upgraded during construction. The Cedar Residence project in Maine used a similar approach — and landed close to the owner's expectations, largely due to strong upfront communication and careful scope management.

A GMP contract is a cost-plus contract with a ceiling. The contractor agrees not to exceed a set maximum amount — though a contingency buffer for unknowns is typically included.

To incentivize efficiency, any savings below the GMP are often shared between owner and contractor. This aligns everyone's interests around finding smart cost-saving solutions without compromising quality.

The more detailed your house plan construction drawings, the more accurate your contractor's GMP will be — and the smaller the contingency buffer they'll need to add. Explore our detailed floor plans →

How GMP savings sharing works
If the concrete foundation is budgeted at $100,000 but completed for $70,000, the $30,000 savings might be split 50/50 — $15,000 returned to the owner, $15,000 kept by the contractor as a bonus. This motivates the contractor to source materials efficiently, optimize the construction sequence, and work closely with trades for better pricing.

In a design–build contract, the designer and contractor operate as a single team under one contract. This streamlines communication and can significantly compress the project schedule.

The tradeoff: because the builder holds the primary contract, the designer reports to the contractor rather than directly to you as the owner. This can shift design decisions toward what's easiest to build rather than what best serves your vision.

Design quality varies widely between firms. Some design–build companies prioritize speed and efficiency over design detail. Others — particularly those led by architects — maintain strong design standards while using the integrated delivery model to move faster.

In some cases, the owner acts as their own general contractor — bidding each trade package separately and managing coordination directly. This structure is most common on large projects with significant manpower needs, or when cost control is the top priority.

The owner bids each phase independently — foundation, framing, mechanical, electrical, plumbing — and oversees scheduling and sequencing between trades.

Important
This approach only works when the owner has direct experience in construction management, or when a third-party owner's representative or project manager is hired to handle day-to-day coordination. Without that expertise, the cost savings of cutting out a general contractor are quickly lost to scheduling gaps, subcontractor disputes, and budget overruns.

How to choose the right builder contract for your custom home