6. How This Differs from Conventional Estimating Approaches

Most estimating and QS software — and most manual feasibility processes — are built around the same basic sequence: a design or drawing exists, quantities are measured or taken off from it, and rates are applied to those quantities. Established platforms such as RIB Candy and RIB CostX are well regarded for exactly this: first-principles, resource-based estimating and BIM/2D takeoff that turns a measured design into a detailed, auditable bill of quantities and cost plan. The same is true of a benchmark-based QS workflow or a manually built spreadsheet feasibility model — all of them start from a design (or a benchmark standing in for one) that can be measured.

The gap those tools do not address is the stage before a design exists to measure. At concept and feasibility stage there is no drawing to take off — only a set of assumptions about site, form, and structure. The Global Variables methodology is built for that earlier stage: quantities are generated from formulas linked to a small set of variables, rather than measured from a design, which is what allows a feasibility figure to be produced, and a design alternative to be tested, before there is anything to take off. The two approaches are complementary rather than competing — a GV feasibility model does not replace a measured, tendered estimate once a design exists; it gives that later process a more transparent and better-tested starting point.

This is a difference in structure and in the stage of the project each is suited to, not a claim that one replaces the other. Because GV quantities, programme, and cost are derived from the same underlying variables rather than assembled from separate inputs, they stay consistent with each other automatically at the feasibility stage — which is the specific gap conventional takeoff-based tools are not designed to fill.

7. Cost Substantiation: A Resource-Based Build-Up

The output of the Global Variables model is not a single derived number. Each budget is supported by a resource-based estimate typically comprising several hundred individual line items — in practice, more than 500 on a typical project — covering:

• Labour resources, by trade and productivity assumption
• Material quantities and rates
• Plant and equipment allowances
• Subcontractor packages
• Consultant costs
• Temporary works and site overheads
• Risk provisions and project-specific adjustments

The distinction this creates is between a budget presented as a conclusion and a budget presented as a calculation. A stakeholder reviewing the estimate can trace a given cost back to the quantity and rate that produced it, and from there back to the Global Variable and formula that generated the quantity. This does not make the estimate more accurate by itself — accuracy still depends on the quality of the rates and productivity data behind it — but it does make the estimate reviewable in a way that a single headline figure is not. Appendix A shows a short, illustrative extract of what that line-item detail looks like.

7.1 How resources are expressed

Each resource in the build-up is expressed in the unit appropriate to how it is actually consumed on site, rather than as a single blended rate. In broad terms:

• Materials are quantified by volume, area, length, or count as appropriate — for example, concrete in cubic metres, formwork in square metres, reinforcement in tonnes
• Labour is quantified in hours, derived from the resource quantity and a productivity assumption (output per day for a given trade and crew size)
• Plant is quantified in hours or days of use, linked to the same activities that consume the labour and material

Expressing resources this way is what allows the same quantities to feed both the cost build-up and the programme: a labour hour figure produces a cost when multiplied by a wage rate, and a duration when divided by a daily output rate. The two are simply different uses of the same underlying number.

8. Traceability and the Audit Trail

Four properties of the model are relevant to how it is used in practice:

In practice, reviewing an estimate means walking the dependency chain from Section 5 in reverse: starting from a cost of interest, identifying the resource and quantity behind it, and from there the formula and Global Variable that generated it. That reverse path — cost, to resource, to quantity, to formula, to Global Variable — is the audit trail. It allows a specific figure to be questioned and checked against a specific assumption, rather than the estimate being accepted or rejected as a whole.

Traceability is a property of the model’s structure, not a guarantee of accuracy. The reliability of any output still depends on how well the underlying rates, productivity assumptions, and formula logic reflect the market and project type in question, and those inputs should be reviewed and calibrated by a qualified estimator or quantity surveyor for the relevant sector before the output is relied upon for investment decisions.

9. Scenario Comparison

Because outputs are recalculated from variables rather than rebuilt from scratch, the model supports comparison of alternative project configurations within the same working session. Typical variables tested this way include building height, floor count, site dimensions, structural system, foundation strategy, and building use mix. Each change updates quantities, programme duration, consultant fees, resource requirements, and total project cost together, which allows a design or investment team to compare several configurations on a consistent basis before committing to detailed design.

This is most useful at the point where a design decision is still genuinely open — for example, comparing a concrete frame against a structural steel alternative, or testing whether an additional floor is viable within a fixed budget. Once a scheme has progressed into detailed design, the value of rapid scenario comparison naturally decreases, since the project has moved from a small number of open variables to a large number of fixed design decisions.

9.1 Illustrative example

The specific percentages and durations above are illustrative only — real figures depend on local rates, market conditions, and project-specific factors, and should not be read as a general claim that steel outperforms concrete. The point of the comparison is structural: changing a single Global Variable (structural system) automatically restates structural cost, programme duration, preliminaries, and financing cost together, so a trade-off of this kind becomes visible within one model rather than requiring two separately built estimates to compare.

10. Programme Integration

In most feasibility work, the cost plan and the construction programme are produced as separate exercises, often by different people, and reconciled — if at all — only at a high level. A change to the design is applied to the cost plan and to the programme independently, and the two can drift out of alignment without anyone noticing until much later.

Because the resource quantities behind the cost build-up (Section 7.1) are the same quantities used to derive programme durations, the two cannot drift apart in the same way here: a change to a structural quantity changes both the cost and the duration in the same step, because both are derived from it. Preliminaries and site overheads, which are themselves a function of programme duration, and financing cost, which is a function of both duration and drawn-down capital, follow the same path. The practical effect is that the interaction between a longer, cheaper option and a shorter, more expensive one — as in the illustration above — is visible directly, rather than requiring a separate reconciliation exercise between the estimating and planning functions.