ENGINEERING PRACTICE | 25 AUGUST 2026
When Site Conditions Differ from the Design: Who Makes the Decision?
Understanding the roles of the designer, supervising engineer and contractor when construction meets reality.
Construction drawings represent the engineer’s considered solution based on the information, investigations, design assumptions and requirements available at the time of design. They are essential documents, but they cannot reproduce every condition that will eventually be encountered on site.
Excavation may expose unexpected ground conditions. Existing structures or services may differ from available records. Dimensions may not correspond exactly with those assumed during design. A detail that appeared straightforward on paper may prove difficult to construct when confronted with the physical realities of the site.
When this happens, an important question arises: Who decides what should be done?
The answer is not simply “the designer,” “the supervising engineer,” or “the contractor.” Each has a distinct professional role. The important issue is knowing where one responsibility ends, where another begins, and when a matter must be referred back for design review.
Managing these situations properly is an important part of professional construction supervision. Poorly managed, they can lead to delays, disputes, unauthorised changes and, in the worst cases, compromised structural performance.
1. The Design Is the Starting Point, Not the End of the Story
A structural design is developed from defined information: architectural drawings, geotechnical investigations, surveys, loading requirements, applicable standards and other design inputs. The designer uses these to develop a safe and practical structural solution.
Once construction begins, however, the site starts providing additional information.
The contractor may uncover conditions that were not apparent during design. Dimensions may vary. Existing construction may differ from record drawings. Ground conditions can change within relatively short distances. Services may appear where none were expected.
These discoveries do not automatically mean that the original design was defective. They mean that the project has encountered new information that must be assessed and managed appropriately.
2. The Supervising Engineer Is the First Line of Technical Decision-Making
The supervising engineer occupies a particularly important position because he or she is closest to the construction process and is expected to interpret the drawings in the context of actual site conditions.
Many issues encountered during construction do not require redesign. They require engineering judgement: interpreting a detail, resolving a minor dimensional discrepancy, coordinating structural work with architectural or services requirements, or determining how an approved detail should practically be executed.
These are ordinarily matters for the supervising engineer to assess and resolve on site.
However, there is an important boundary. Where the proposed solution changes the structural concept, load path, member size, reinforcement, foundation arrangement or another fundamental design assumption, the matter should be referred back to the designer for review.
The distinction is therefore not between “important” and “unimportant” site decisions. It is between decisions that implement the design and decisions that change the design.
3. The Contractor Must Identify and Report, Not Redesign
The contractor is usually the first party to encounter a discrepancy or unexpected condition because the contractor is physically executing the works.
When something does not correspond with the drawings, the contractor has an important responsibility: identify the issue, document it and bring it to the attention of the supervising engineer.
The contractor may also propose a practical solution. Contractors often possess considerable construction experience, and their proposals can be valuable. However, proposing a solution is different from authorising it.
A contractor should not independently alter structural dimensions, reinforcement, member arrangements or other design requirements simply because an alternative appears easier to construct.
Where clarification or alteration is required, the appropriate instruction should come through the project’s established technical and contractual channels.
This protects everyone involved. The contractor avoids responsibility for an unauthorised alteration, the supervising engineer retains control of the works, and the designer is consulted where the proposed change affects the design itself.
4. When Must the Matter Go Back to the Designer?
Not every site problem needs to return to the original designer. If it did, construction would become unnecessarily slow and the role of the supervising engineer would be reduced to little more than relaying questions.
However, some matters clearly cross the boundary from supervision into design.
The designer should normally be consulted where the proposed action affects the structural concept or the assumptions on which the design was based. Examples include changes to member sizes, reinforcement, structural layout, load paths, foundation systems, design loads or other matters that could materially affect structural behaviour.
The same applies where site conditions reveal that an important design assumption was incorrect. Unexpected ground conditions, significant dimensional discrepancies or previously unknown structural constraints may require the designer to reconsider part of the original solution.
The practical test is simple:
Does the proposed decision merely enable the approved design to be constructed, or does it change the engineering basis of that design?
If it changes the engineering basis, the designer should be brought back into the decision.
5. A Practical Decision Pathway
When a discrepancy or unexpected condition is encountered on site, the following sequence provides a useful approach:
1. Identify the issue.
The contractor records what has been found and compares it with the drawings, specifications or other relevant documents.
2. Refer it to the supervising engineer.
The supervising engineer assesses the condition and determines whether it can be resolved within the intent of the approved design.
3. Determine whether the proposed solution changes the design basis.
If it is simply a matter of interpretation, coordination or practical implementation, the supervising engineer can ordinarily resolve it.
4. Refer genuine design changes to the designer.
Where structural behaviour, design assumptions, member sizes, reinforcement, loading, foundations or other fundamental design matters are affected, the designer should review the proposed change.
5. Document the decision.
The final instruction should be recorded through the appropriate project documentation before the affected work proceeds.
This creates a very simple professional chain:
Contractor identifies → Supervising Engineer assesses → Designer reviews where necessary → Decision is documented → Work proceeds.
6. Why the Distinction Matters
Clear separation of professional responsibilities is not about protecting territory. It is about ensuring that decisions are made by the person best placed and properly authorised to make them.
If every site issue is referred back to the designer, supervision becomes ineffective and construction can be unnecessarily delayed. Conversely, if significant design changes are made on site without reference to the designer, the integrity of the original engineering solution may be compromised.
Good project management therefore requires both professional judgement and professional restraint.
The supervising engineer must be confident enough to make legitimate site decisions, but equally able to recognise when a matter has moved beyond supervision and requires design input.
Likewise, the designer should remain available when genuine design questions arise, without becoming the de facto supervisor of works for which another professional has been appointed.
Ultimately, successful construction depends not on one professional making every decision, but on each professional making the right decisions within the right sphere of responsibility.
Conclusion
Construction inevitably brings conditions that could not be fully anticipated at the design stage. The professional response is not to treat every discrepancy as a design failure, nor to allow uncontrolled changes on site.
The contractor should identify and report the issue. The supervising engineer should assess it and resolve matters that properly fall within supervision. Where the proposed action changes the engineering basis of the design, the designer should be consulted.
The drawings start the conversation with the site. Good engineering judgement completes it.
Eng. Benson Kioko Muange
Civil/Structural Engineer
Managing Director, Mutwii Engineering & Management Consultants Ltd.
25 August 2026