Design-build, bridging and progressive design-build

In design-build, the owner signs one contract with one entity, the design-builder, which is responsible for both designing and constructing the project. The owner describes what it needs, usually in performance terms; the design-builder decides how to provide it, produces the design, and builds it. When something goes wrong with the design or the construction, there is one party to answer for it. This single point of responsibility is the method's defining feature and the reason it has spread from private industrial work into highways, transit, water treatment, military bases and public buildings in both countries.

How design-build is organized

Most design-builders are led by contractors, which hire the architect and engineers as subconsultants. Some are joint ventures between a contractor and a design firm, and a few are led by designers who subcontract the construction. In every arrangement the owner deals with one contracting party. The owner still needs its own professional advisers: someone must write the owner's requirements, review the design-builder's design for compliance, and administer the contract. Owners who skip that step lose control of quality.

The owner's requirements

In design-build, the document that matters most is the one the owner writes before procurement: the owner's statement of requirements, also called the owner's project requirements, design criteria package or request for proposals technical requirements. It describes function, performance, quality, codes and standards, mandatory design elements and the site, without designing the project. Writing it well is hard. Too vague, and the owner receives a building that meets the words but not its needs. Too prescriptive, and the owner loses the innovation and price competition that design-build offers and takes back design risk for the parts it specified.

A useful discipline is to ask, for each requirement, whether the owner truly needs to control the solution, or only the result. Many public owners include a reference design to show that a compliant solution exists, while stating that bidders may propose alternatives and that the reference design carries no warranty.

Bridging

Bridging is a hybrid in which the owner hires a bridging architect or engineer to take the design to about 20 to 35 %, enough to fix the layout, appearance and major systems, and then procures a design-builder to complete the design and build it. The owner gains control over the elements it cares most about while still transferring detailed design and construction to one party. The risk is that the boundary between the owner's design and the design-builder's design becomes the source of claims, because each side can argue the other's documents caused a problem. Clear contract language on who is responsible for errors in the bridging documents is essential.

Selecting a design-builder

Because bidders on a design-build project must design as well as price, proposing is expensive. Public owners usually use two steps: a request for qualifications to shortlist three to five teams, followed by a request for proposals in which the shortlisted teams submit technical proposals and prices. Many owners pay unsuccessful shortlisted teams a stipend, often a fraction of a percent of the contract value, in exchange for the right to use their ideas. Award may go to the lowest price among technically acceptable proposals, to the best value combining price and technical score, or, in a fixed-price best-design approach, to the best design for a stated price.

Canadian public owners use two-stage design-build procurements under their normal procurement policies. Provincial transportation ministries, municipal water utilities and federal departments use design-build for highways, bridges, treatment plants and buildings, and provincial agencies such as Infrastructure Ontario list design-build projects alongside partnership projects in their market updates.

# Comparing award rules for three design-build proposals (illustrative)
props = {"A": (86, 61.2), "B": (78, 57.9), "C": (91, 66.0)}   # (technical score out of 100, price $M)
low_price = min(props, key=lambda k: props[k][1])
adjusted = min(props, key=lambda k: props[k][1] / props[k][0])
# weighted: 60 % technical, 40 % price, price points = 40 x lowest price / price
best = min(p for _, p in props.values())
weighted = {k: 0.6 * t + 40 * best / p for k, (t, p) in props.items()}
w_win = max(weighted, key=weighted.get)
print("Lowest price wins:   ", low_price)
print("Adjusted score wins: ", adjusted, {k: round(p / t, 3) for k, (t, p) in props.items()})
print("Weighted score wins: ", w_win, {k: round(v, 1) for k, v in weighted.items()})
stipend = 0.002 * 60_000_000
print(f"Stipend at 0.2 % of a $60M contract: {stipend:,.0f} per unsuccessful team; two teams cost {2 * stipend:,.0f}")

Lowest price wins:    B
Adjusted score wins:  A {'A': 0.712, 'B': 0.742, 'C': 0.725}
Weighted score wins:  C {'A': 89.4, 'B': 86.8, 'C': 89.7}
Stipend at 0.2 % of a $60M contract: 120,000 per unsuccessful team; two teams cost 240,000

Different award rules can pick different winners from the same proposals, which is why the rule must be published in the request for proposals and applied exactly. Weighted scoring in particular is sensitive to how price points are calculated; small changes in the formula can reverse a result.

Progressive design-build

Traditional design-build fixes the price at award, on the basis of the owner's requirements and the proposer's early design. That works when requirements are clear, but on complex or uncertain projects the design-builder must price a great deal of risk it cannot yet see. Progressive design-build, which has grown rapidly since the 2010s, especially in water, wastewater and complex buildings, splits the contract into two phases.

Progressive design-build gives the owner more influence over the design and a price based on much better information, at the cost of losing price competition on the construction phase. It looks a lot like CMAR, with the difference that the design-builder, not the owner, holds the design contract and carries design responsibility. Owners use independent cost estimators and open-book reviews to protect themselves in the negotiation.

Design responsibility

Who carries design risk in design-build depends on the contract. Usually the design-builder warrants that its design will meet the owner's requirements, and it carries the cost of correcting design errors. Its designers, as subconsultants, are typically liable to the design-builder only for failing to meet the professional standard of care. That creates a gap: the design-builder may have promised the owner fitness for purpose while its designer promised only reasonable care. Design-builders manage that gap through design contingency, professional liability insurance, and careful wording of the performance guarantees they give.

The owner's own reviewers must be careful not to take design risk back. If owner reviewers direct design changes or approve deviations from requirements, the design-builder may argue the owner assumed responsibility for the result. Good design-build contracts say that owner review is for general conformance only and does not relieve the design-builder of its obligations.

Standard contracts

Owners on large public projects often write their own design-build contracts, especially for highways and transit, rather than using industry forms. The Design-Build Institute of America's forms are widely used in the United States because they were written specifically for design-build, including progressive design-build.

Where design-build works

Design-build works best when the owner can state its needs clearly in performance terms, values speed and single responsibility, and is willing to let the design-builder make design decisions. Highways, bridges, parking structures, warehouses, standard schools, treatment plants and repetitive facilities fit well. It struggles where the owner wants to control design details closely, where requirements are uncertain or likely to change, and where the owner's review process turns into redesign. Progressive design-build addresses some of those weaknesses for complex projects.