A RIBA stage cost plan is not one document produced repeatedly with improving accuracy. At each stage it answers a different question, and the useful test is whether it answers the question actually being asked at that point.
Where it does not, the symptom is familiar: a cost plan that says the same thing at stage 3 as it did at stage 1, only with more decimal places.
Stage 0 and 1, is this worth doing
The question is whether the scheme is viable and what the scale of commitment is. The document is an order of cost estimate built from an area schedule and whatever is known about the site.
What it must do is identify the items likely to move the answer, with values, rather than producing a single total. At this point the assumptions carry more information than the arithmetic.
What it cannot do is give a price. Anybody treating a stage 1 figure as a budget is going to have a difficult conversation later, which is covered in what an order of cost estimate should contain.
Stage 2, does the concept work commercially
The question becomes which option to pursue. The cost plan should be able to compare alternatives on a consistent basis: massing options, structural approaches, unit mix variations, tenure splits.
That requires the plan to be structured so that a change can be isolated. A total that has to be rebuilt from scratch to test an option is not fit for the stage.
It also requires the abnormals and the site costs to be separated from the building, because option comparison at this stage is frequently decided by them rather than by the superstructure.
Stage 3, is the design affordable
Now there is enough information for an elemental cost plan proper, and the question is whether the design as developed sits inside the budget set earlier.
The key output is not the total but the reconciliation: what has changed since the last issue, why, and by how much. Growth attributable to design development is different from growth attributable to a change in scope, and separating them tells the client whether they have a design problem or a brief problem.
Risk should reduce here. Design development risk falls as details are resolved, and a plan showing the same allowance as stage 2 has not been properly reviewed.
Stage 4, will the tender come in
The question is whether the documented scheme will price at the expected level. The cost plan converges towards the form the tender will take, package by package where relevant.
This is the stage where market testing has most value: checking key packages with the supply chain before the tender goes out, rather than discovering the position when bids return.
It is also the last stage where a problem is cheap to fix. After tender, correcting a cost position means either changing scope with a contractor already appointed or accepting the number.
Reconciliation is the thread
Every issue should reconcile to the previous one. Opening figure, changes listed with values and reasons, closing figure. Nothing else builds confidence in the same way.
Without it, each issue is a new number and the client has no basis for judging whether the cost is being managed or merely reported. With it, a rising cost plan can still be a well controlled project, because every increase has an owner and a decision behind it.
On mixed use schemes the reconciliation has to work by use as well as in total, which is why the structure has to be set at the first issue rather than added later. That is set out under mixed use and regeneration.
Where the stages break down
The common failure is a scheme moving to tender at stage 3 information because the programme demands it. The cost plan is then asked to carry a certainty the design does not support.
The honest response is to say so, quantify the exposure through provisional sums and risk, and let the client decide with the position visible. The dishonest response is a confident number.
That failure and its consequences are described in incomplete design information.
What this means for you
At each issue, ask what question this cost plan answers that the last one could not. If there is no answer, the document has been updated rather than developed.
Then ask for the reconciliation. It is the single most useful page in a cost plan and it is the one most often omitted. Our approach sits under cost planning and estimating.
Unsure what your cost plan should be telling you at this stage?
Tell us where the design has reached. We will set out what a cost plan can honestly answer at that point.