How reliable is an estimate at concept stage: margins, uncertainty and what it really commits

The reliability of a construction estimate is directly tied to the quantity and quality of the information available when it is produced. The more advanced the drawings, the more the technical choices are settled, the higher the precision can be. That is a mechanical reality, not a professional shortcoming.

The way precision improves across the stages follows a curve well documented in the practice of construction economics.

The outline-design line is the one that concerns us here: ± 20 % on a project of 1 M€ means a band from 800,000 to 1,200,000 €, a spread of 400,000 € between the two bounds. That is not a defect; it is an honest reflection of what is known about the project at this point.

ℹ Using the band properly A well-advised client does not take the lower bound as the reference budget. They work with the middle of the band as the working assumption and allow a client reserve of 10 to 15 % to absorb the risks the studies do not cover. For example: an outline estimate of 900,000 to 1,200,000 € → working budget 1,050,000 € plus a reserve of 150,000 €, giving a total envelope of 1,200,000 €.

The ± 20 % band of an outline estimate is not arbitrary. It results from the accumulation of several distinct sources of uncertainty, some of which can be reduced at this stage and some of which cannot.

The table repays reading closely: the most impactful uncertainties — ground conditions, hazardous materials, latent architectural complexity — cannot be resolved by the outline estimate alone. They call for further investigation: a ground survey, an asbestos and lead survey, development of the drawings. That is why the outline figure is a band and not a single number.

A properly produced outline estimate engages the economist's professional responsibility for the coherence of their method and the soundness of their assumptions. If those assumptions are explicitly documented — areas, level of specification, assumed ground conditions, tender date — and if the market suffers no unforeseen upheaval between outline design and award, the final figure should not depart significantly from the band announced.

It also engages the design team's credibility with the client: if the outline estimate says 900,000 € and the tenders come in at 1,500,000 €, that is a professional failure which calls the quality of the economic framing into question.

An outline estimate is not a price guarantee. It does not commit the client to delivering the project within that envelope, nor the economist to defending the figure at all costs if the brief changes. Any substantial change of brief — added area, a different structural type, a higher level of specification — invalidates the estimate and requires it to be redone.

ℹ The scenario to avoid absolutely A client who signs a design appointment fixing the lower bound of an outline estimate as the contractual budget condemns themselves to permanent tension with their design team. The contractual budget should rest on the middle or upper part of the band, with an explicit reserve for risk. Projects that exceed their budget by 30 to 40 % are almost always projects where the initial budget rested on an optimistic reading of an outline estimate.

The precision of an outline estimate is directly proportional to the quality of the data given to the economist. The more precise the areas, the better defined the specification, the better known the ground, the narrower the band. Investing in a properly developed sketch design and a preliminary ground survey can take the uncertainty from ± 20 % to ± 15 % at this stage.

An outline brief — "10 apartments of two to four rooms" — produces a less reliable estimate than a detailed one: "10 apartments of 45 to 75 m² habitable, four storeys in reinforced concrete, underground parking, accessible flat roof, intermediate finishes". Each additional precision narrows the economist's assumptions and improves the relevance of the result.

Some uncertainties can be reduced at this stage by targeted, inexpensive investigation: a site visit to spot access constraints, consulting the urban geology mapping to identify ground risk, a land registry check on party wall constraints. These let the economist build identified risks into their assumptions rather than absorb them into a general contingency.

An outline estimate has a limited shelf life. At each stage the economist must update the assessment to take in the new information available. The sequence from outline to developed design to tender is what guarantees a progressive landing on the real cost of the project — provided no step is skipped.

On refurbishment the band of uncertainty is structurally wider than on new build. The reason is simple: the real condition of the existing fabric only emerges during the works. Weakened floors, services at the end of their life, structure not compliant with today's loadings, asbestos in materials the survey did not list — any of these can surface during construction and generate significant extra cost.

On refurbishment a contingency of 15 to 20 % is generally more appropriate than the standard 8 to 10 %. Some economists go so far as to set out two scenarios in their estimate — one where the visible condition proves sound, one where significant discoveries occur — with the gap between them explicitly documented.

ℹ Refurbishment: the right reflexes Before commissioning an outline estimate on a refurbishment: obtain an asbestos and lead survey if none exists, carry out a detailed inspection with opening up of floors and services, and allow a contingency of at least 15 % in the working budget.

You have better things to do. Submit your project on quostra.com and an economist produces your outline estimate with documented assumptions and an honest band.

The reliability of an outline estimate is a question of context: ± 20 % at concept stage is not a defect, it is the precision expected at that point in the project. What determines the quality of an outline estimate is not the width of the band but the rigour with which the assumptions are set, the sources of uncertainty identified and the risks anticipated. A good outline estimate gives the client a clear view of what they are committing to, and of what cannot yet be known.