A two stage tender is not a safer version of a single stage one. It is a different transaction, with a different distribution of risk and a different amount of commercial leverage, and it should be chosen on the facts of the scheme rather than on what the last project did.
Both routes are legitimate. In practice both are frequently chosen for the wrong reason, which is that somebody in the room has a preference and nobody has costed the alternative.
What each route actually is
Single stage means the design is developed to a point where it can be described completely, the documents go to a list of contractors, and they price the same scope in competition. The result is a fixed price for a defined scope, arrived at under market pressure.
Two stage means a contractor is selected early, largely on their team, their preliminaries and their overhead and profit percentages, and is then paid to work alongside the design team while the remaining packages are procured. A price is agreed at the second stage.
The essential difference is when competition happens. In single stage it applies to the whole price. By contrast, in two stage it applies to a fraction of it at stage one, and to the subcontract packages at stage two, with the main contractor's own margin already fixed.
What two stage buys
Buildability input while the design can still change. A contractor reviewing details at the point they can be altered is worth more than one commenting after the price is fixed.
Programme. Early packages can be procured and started while later design continues, which on a long scheme can bring completion forward by months.
Access to a contractor who would not have tendered. On a complex or constrained scheme, the firms best equipped to build it are often the ones least willing to price it blind.
Supply chain engagement. Specialist subcontractors involved early can resolve interface problems that would otherwise become variations.
What two stage costs
Leverage, and the loss is not small. Once a contractor is appointed under a pre construction services agreement, the practical ability to walk away reduces every week. By the time the second stage price arrives, changing contractor means restarting the design coordination and losing the programme benefit that justified the route.
Both parties know this. A second stage price is therefore a negotiation, and negotiations are decided by who can most easily walk. That is why the preliminaries and the overhead and profit percentages have to be fixed competitively at stage one and expressed as percentages that cannot be revisited.
Where they are not fixed, the route delivers the disadvantages of both approaches: no competition on the main contractor's price and no early certainty either.
The condition that decides it
The useful test is whether the design can be described completely enough to price. Not whether it is finished, but whether a contractor could produce a firm price without qualifying it into meaninglessness.
Where it can, single stage is usually better. Competition across the whole price is the strongest commercial tool available and giving it up should require a reason.
Where it cannot, single stage does not produce certainty, it produces the appearance of certainty. Contractors price the uncertainty as risk, qualify the parts they cannot assess, and the difference emerges later as variations. That failure is set out in tender documentation gaps.
Making a second stage price testable
If two stage is the right route, the second stage has to be structured so the price can be tested rather than accepted. That means open book subcontract tendering with the client's surveyor seeing the returns, an agreed number of bids per package, and a stated rule for how packages are selected.
It also means a risk register carried openly rather than a lump sum contingency inside the contractor's price. Where risk sits inside the price it is spent whether or not the risk occurs.
Finally it means a defined point at which the client can decline the price and take the design elsewhere, with the pre construction work owned by the client. Whether or not that right is ever used, having it changes the negotiation.
Phased schemes push towards two stage
Where a project hands over in sections, or where the site is live and the sequence drives cost, contractor input on logistics has more value than on a straightforward scheme, and the programme benefit of overlapping design and construction is larger.
That is a genuine argument for two stage, but it strengthens rather than removes the need to fix the percentages at stage one, because a phased job generates more preliminaries and more change than a simple one. The interaction is set out under complex phasing.
What this means for you
Ask what each route costs in the specific case rather than which is better in general. On a scheme with a complete design, the case for single stage is strong and the burden should be on anybody proposing otherwise.
If you go two stage, fix the percentages competitively at stage one and write down how the second stage price will be tested. Those two decisions carry most of the commercial value in the route, and both are cheap to make at the start and impossible to make later. Our approach is set out under tendering and procurement.
Deciding how to take a scheme to market?
Tell us the stage the design has reached, the programme and how it is funded. We will set out what each route costs you and what it buys.