Why Existing Building Investigations Matter

Existing building investigations often reveal information that cannot be obtained from drawings alone. Understanding how a building actually works is often the most valuable part of the engineering process.

Understanding the Building Before Making the Decision

Published: September 2026

Prepared by: Michael Somerville, CPEng, IntPE, CMEngNZ

When owners engage an engineer, there is often an expectation that the next step will be calculations, seismic ratings or strengthening recommendations.

In reality, some of the most important engineering work happens before any calculations begin.

The first step is often understanding the building itself.

Good decisions start with understanding the building, not just analysing it.

For existing buildings, the challenge is rarely a lack of engineering solutions. It is often a lack of reliable information.

Why Existing Buildings Are Different

Unlike new buildings, existing buildings come with history.

Over time, buildings may have:

• Undocumented alterations

• Removed walls or columns

• Changes in occupancy

• Additions and extensions

• Repairs and upgrades

• Strengthening work that may or may not have been completed

• Missing drawings and records

Because of this, understanding how a building works today is often more important than understanding how it was originally designed.

What Does an Existing Building Investigation Involve?

Foundation investigations can help confirm load paths, structural assumptions and the reliability of existing construction details.

An existing building investigation may include:

• Reviewing historical drawings

• Reviewing building consent records

• Reviewing previous engineering reports

• Site inspections

• Verification of structural elements

• Investigation of alterations

• Assessment of load paths

• Identification of critical structural systems

The objective is not simply to gather data.

The objective is to understand:

How does this building actually work?

Why Drawings Don't Always Tell the Full Story

Many existing buildings have undergone decades of alterations, repairs and upgrades. Site investigations help confirm what is actually present rather than relying solely on historical records.

Historical drawings and records provide valuable clues, but site investigations are often required to confirm how existing structural systems actually behave.

One of the most common discoveries during existing building investigations is that the building on site does not perfectly match the available drawings.

For example:

• Walls may have been removed.

• Openings may have been enlarged.

• Structural members may have been altered.

• Previous strengthening may not have been fully documented.

• Successive renovations may have changed the original structural system.

This is why physical inspection and verification remain important.

A Real-World Example

A recent detailed assessment of an operational building demonstrates why investigations matter.

An earlier screening-level assessment recorded an estimated rating of approximately 71%NBS. That provided a useful indication of the building's performance but offered limited information about the structural mechanisms governing the outcome.

A later Detailed Seismic Assessment reviewed:

• Structural systems

• Load paths

• Roof-to-wall connections

• Wall behaviour

• Foundations

• Building geometry

• Overall load transfer mechanisms

The assessment found that the masonry wall system had significant reserve capacity relative to the reviewed demand assumptions. The critical engineering issue was not wall strength, but understanding foundation restraint mechanisms and load-path continuity.

The most valuable outcome was not a new percentage.

The most valuable outcome was a better understanding of how the building actually behaved and where future engineering effort should be focused.

Why Investigations Can Save Money

Owners sometimes view investigations as an additional cost.

However, investigations can often:

• Reduce uncertainty

• Avoid unnecessary strengthening

• Identify reserve structural capacity

• Clarify where investment should be directed

• Improve decision-making

• Reduce redesign during construction

Better information often leads to more proportionate engineering decisions.

When Is an Investigation Worthwhile?

An existing building investigation may be worthwhile when:

Buying a Building

To understand structural condition, existing documentation and potential future expenditure.

Reviewing a Seismic Assessment

To better understand assumptions, vulnerabilities and opportunities.

Planning Alterations

To identify how the building currently works before modifications occur.

Change of Use

To understand whether the existing structure can support future requirements.

Asset Planning

To support long-term refurbishment, redevelopment or retention decisions.

Common Misconception

"The engineer already has the drawings, so they know how the building works."

Not necessarily.

Drawings provide valuable information, but they do not always capture:

• Subsequent alterations

• Construction differences

• Building deterioration

• Previous repair work

• Actual structural behaviour

The purpose of an investigation is to confirm how the building exists today.

The Better Question

Rather than asking:

"What is the rating?"

consider asking:

"How well do we understand the building?"

The answer to that question often determines the quality of every decision that follows.

Key Takeaways

✅ Existing buildings often contain unknowns.

✅ Drawings do not always tell the whole story.

✅ Investigations help confirm how a building actually performs.

✅ Better information can reduce uncertainty and improve decision-making.

✅ Understanding the building is often more valuable than immediately calculating a rating.

Need Advice on an Existing Building?

Whether you're reviewing historical engineering information, considering alterations, assessing an existing building or planning future investment, Essen Engineering can help you understand the building and identify practical next steps.

Discuss Your Building With Us

Building Owner Insights Series

Part 1: Understanding Changes to New Zealand's Earthquake-Prone Building System

Part 2: What Does a %NBS Rating Really Mean?

Part 3: When Is a Seismic Assessment Worthwhile?

Part 4: Five Questions Every Commercial Property Owner Should Ask

Part 5: My Building Has a Low %NBS Rating. What Are My Options?

Part 6: How to Work Effectively With a Structural Engineer

Part 7: Why Existing Building Investigations Matter

Legislative and Professional Disclaimer

Last Updated: 12 October 2026

This article provides general information only and does not constitute building-specific engineering, legal, financial, insurance or investment advice. Building owners should obtain professional advice specific to their property before making decisions regarding compliance, strengthening, alteration, redevelopment or investment.