Why Construction Costs Change After Building Work Starts

Finding out mid-build that the project will cost more than the original quote is one of the most common and stressful experiences in residential construction. It is also one of the most preventable, at least in part.

Some cost changes on a construction project are legitimate. Ground conditions are not fully knowable before excavation begins. Material prices move. Structural discoveries during renovation can require work that could not have been anticipated before the old structure was opened. A client who changes their mind about the floor plan mid-build creates real additional costs.

Other cost changes are the direct result of an inadequate scope, an incomplete set of drawings, a contract that made no provision for how changes would be handled, or a budget built on an outdated price schedule. These are avoidable. The question is usually not whether they will happen, but whether the client will be in a position to tell the difference when they do.

What a Variation Actually Is

In construction contract language, a variation is any change to the agreed scope of work. This can mean additional work, reduced work, substituted work, or a change to the specification of materials or finishes.

A variation order is the formal instruction that authorises a variation, records what is changing, and establishes what effect the change has on the contract price and programme. Under properly drafted construction contracts, variations can only be instructed in writing and must be priced before the work proceeds.

In practice, on many Zimbabwean residential builds, variations happen verbally. A client visits the site and asks for an extra room window. The contractor adds it. No formal instruction is issued. No price is agreed. The cost appears on the next payment request and the client disputes it because they were not told it would cost more.

The absence of a written variation procedure is one of the most reliably expensive choices a homeowner can make.

The Main Causes of Cost Change After Work Begins

Client-initiated scope changes

A client asking for something different from what was specified is the most visible cause of construction cost changes. According to a 2025 study published in Engineering, Technology and Applied Science Research, design changes are the leading trigger for variation orders and contribute to 56.5% of cost overruns and 40% of project delays on large construction projects. A 2024 study in MDPI's Buildings journal found that variation orders are implicated in over 60% of construction disputes globally.

The causes are usually straightforward: the client sees the walls going up and realises the room layout no longer suits them, changes the kitchen specification after tiles have been quoted, or adds a structure that was not in the original scope. Each of these decisions is legitimate. Each generates additional cost that was not in the original quote.

The problem is not the decision. The problem is that no formal process exists to price the decision before it is implemented, creating a dispute when the bill arrives.

Incomplete scope and omissions in the original quote

If the scope of work was not precisely defined before quoting began, the quote did not describe a complete project. It described a partial one.

Common omissions from residential construction quotes in Zimbabwe include ceiling installation, perimeter wall and gate, external plastering where it was not specified, gutters and downpipes, paving, solar installation, borehole, and septic tank where no sewer connection exists. When these elements are later required, they are added to the project as variations, even though they were always going to be needed.

For a detailed explanation of what a construction quote should contain and what is commonly left out, see What Should Be in a Construction Quote in Zimbabwe?

An omission is not the same as a deliberate deception. Many contractors quote what they know is required for the structure and leave finishing and external work for separate discussion. The risk for the client is treating a partial scope quote as the total project cost.

Unforeseen site conditions

Some cost changes are genuinely not predictable from the surface. Ground conditions in particular are only fully known once excavation begins.

A stand that looks flat and stable at site inspection may have:

• Loose, sandy or unstable fill material requiring deeper founding
• Rock close to the surface requiring mechanical breaking or controlled blasting
• High water table that complicates foundation work
• Old service trenches or buried structures
• Soft ground or expansive clay that changes the structural design requirements

When these conditions are found, the original quoted quantities for excavation and foundations need to change. On residential builds, this typically adds cost rather than reduces it, because the discovery usually requires more concrete, more reinforcing steel, or deeper excavation than was planned.

A 2022 study of Zimbabwe's construction industry by quantity surveyors Tirivavi Moyo and Benviolent Chigara, published in the Journal of Construction Project Management and Innovation, identified unforeseen ground conditions as a recognised cause of cost overruns on construction projects in Zimbabwe. The same study found that poor pre-contract planning, including inadequate site investigation, was the single largest grouped component of cost overrun risk, accounting for over 34% of the total explained variance in cost overrun causes.

On renovation projects specifically, opening an existing structure can reveal structural problems not visible from outside: corroded lintels, failed ring beams, inadequate founding depth, or damp damage hidden behind plaster. These discoveries are legitimate grounds for cost adjustment. They are also, in part, manageable through a thorough pre-renovation structural assessment before quoting begins.

Material price movements during the build

In Zimbabwe's current construction environment, material prices do not stay fixed over the length of a build. This is confirmed by published ZIMSTAT data.

According to ZIMSTAT's Civil Engineering Materials Price Index (CEMPI), diesel rose more than 35% in a single month in March 2026, before partially easing by July. Diesel directly affects transport costs for every material delivered to site. ZIMSTAT's Building Materials Price Index (BMPI) shows wall tiles at approximately 60% above their December 2022 level in USD terms, with roofing materials up 22% and timber up 18.5% over the same period.

This data is cited in the Propertyzone construction cost guide reviewed by Dennis Nkomo.

A quote prepared at January 2026 prices for a build that does not start until July 2026 is working from a stale price schedule. If the contract does not address how material price movements are handled during the construction period, one party will bear the entire cost of any movement. In most informal Zimbabwean residential contracts, that party is the client.

Long project duration in an unstable economy

The Zimbabwe QS research by Moyo and Chigara identified currency exchange rate unpredictability as the single most critical cause of cost overruns on construction projects in Zimbabwe, ranked first by both consultants' and contractors' quantity surveyors (RII=0.894 out of 1.0). Unstable economic climate ranked third (RII=0.851) and excessive increases in material and labour prices ranked joint fourth (RII=0.843).

The implication for a residential client is specific. A build that takes six months rather than three months is exposed to twice as much economic movement. A build that runs for twelve to eighteen months because of a stop-start cash flow approach is highly exposed to currency shocks, material price movements, and shifts in contractor rates. The same Zimbabwe research found that long project duration was a recognised risk component contributing to cost overruns.

This is one of the clearest arguments for ensuring that material procurement and funding are in place before construction begins, rather than releasing resources month by month. A short, continuous build carries far less exposure to Zimbabwe's economic environment than an extended, interrupted one.

Excessive use of provisional sums and PC items

A quote that allocates large portions of the project cost to provisional sums and prime cost (PC) items is presenting a headline figure that may be significantly lower than the final cost. As the Zimbabwe QS research found, excessive use of prime cost and provisional sums was identified as a distinct component of cost overrun risk, named specifically as Component 10 in their factor analysis.

Provisional sums and PC items are legitimate where work genuinely cannot be specified in advance. The risk is when they are used to create a competitive-looking headline price, with the real cost emerging as each provisional item is replaced by an actual cost during construction.

For an explanation of what provisional sums and PC items are and how they affect a quote comparison, see BOQ Explained: What a Bill of Quantities Means When Building a House.

What the Research on Zimbabwe Construction Projects Found

The 2022 peer-reviewed study by Moyo and Chigara surveyed 51 quantity surveyors from construction companies and consultancy firms across Zimbabwe. The three most critical causes of cost overruns they identified were:

1. Currency exchange rate unpredictability
2. Poor financial planning for the project
3. Unstable economic climate

Client-initiated change orders ranked ninth on the list. Poor budget estimation ranked eighth. Inadequate design specifications ranked eleventh. Excessive use of prime cost and provisional sums ranked twenty-third.

The picture this creates for a residential homeowner is clear: the leading causes of cost overruns in Zimbabwe's construction environment are not primarily about contractors adding unauthorized charges. They are primarily about the operating environment, the quality of planning that precedes construction, and the financial management of the project while it runs.

A separate 2019 Zimbabwe construction study also identified additional work (variations) and improper planning as consistent top-ten causes of cost overruns. Both studies converge on the same pre-construction problem: insufficient definition of scope, budget and risk before work begins.

What to Establish Before Construction Begins

Accepting that some cost changes are unavoidable, the practical question is what can be done before construction starts to reduce the avoidable ones.

Complete drawings before quoting

A quote taken from preliminary drawings is an estimate, not a fixed project price. As construction quantity methodology research confirms, a BOQ or cost estimate can only be as reliable as the drawings it was taken from. Finalising the design before requesting quotes reduces the scope uncertainty that generates variations later.

Define the full scope including all finishes and external works

Work through the project from site clearance to handover and confirm whether ceiling, perimeter wall, paving, drainage, solar, borehole, and service connections are included or excluded from the quote. Obtain a written list of exclusions from every contractor quoting.

Agree a written variation procedure before work starts

The procedure should require any change to scope to be:

• Instructed in writing
• Priced by the contractor before the work proceeds
• Approved by the client in writing before the work proceeds
• Added to the contract total as a formal variation order

A variation that has not been priced and approved before it is instructed leaves both parties in a position where disagreement is almost certain.

Use a contingency that reflects the actual risk

The Propertyzone construction cost guide recommends a 15% contingency as the minimum appropriate for a Zimbabwean residential build in the current environment. This is not a buffer for scope additions. It is a financial reserve for legitimate unknowns: ground conditions, price movements, and minor variations that arise from site conditions.

Treating the contingency as part of the usable budget defeats its purpose. If there are no surprises, the contingency is simply unspent.

Confirm price validity before committing

A quote or BOQ has a price validity period. Material prices in Zimbabwe move often enough that a quote prepared three or four months before construction starts may not reflect the rates a contractor can achieve when they actually go to market. Confirm with the contractor that the rates still hold before signing a contract and before releasing funds.

Pre-commit key material quantities where possible

For clients who can fund forward procurement, locking in the major structural material quantities (cement, brick, reinforcing steel, roofing sheets) at the start of construction removes much of the price-movement risk from those items. This is particularly relevant for diaspora-funded builds where the client cannot respond quickly to on-the-ground price changes.

What Good Project Management Does for These Risks

Active project management during construction does not prevent cost changes from occurring. What it does is ensure that changes are identified early, priced formally, and approved before they become disputes.

A project manager who monitors the programme and costs regularly can flag when a build is drifting from the agreed scope, when material prices need to be reviewed, and when a site discovery has implications for the budget before it becomes a crisis.

NiceBuilt's project management service covers planning, site coordination, progress monitoring, and quality checks through the construction programme. For a project where cost control matters, coordinating these functions from the start of construction gives the client an informed position throughout.

Learn more about NiceBuilt's project management service

Sources

1. Moyo, T. and Chigara, B. "Causes of cost overruns on Zimbabwe's construction infrastructure projects." Journal of Construction Project Management and Innovation, 12(1): 65–86. University of Johannesburg Press. December 2022. [Peer-reviewed survey of 51 Zimbabwe QS professionals; ranked causes of cost overruns; factor analysis identifying 10 components including currency exchange rate unpredictability (ranked #1, RII=0.894), poor financial planning (#2), client-initiated changes (#9), and excessive use of PC/provisional sums (Component 10)].
2. Moyo and Chigara (2022) drawing on: Oyewobi, L.O., Jimoh, R., Ganiyu, B.O. and Shittu, A.A. "Analysis of causes and impact of variation order on educational building projects." Journal of Facilities Management, 14(2): 139–164. 2016; for increases in construction costs as the most frequent effect of variations.
3. Engineering, Technology and Applied Science Research (ETASR). Study confirming design changes contribute to 56.5% of cost overruns and 40% of project delays on large construction projects. Cited via Xpedeon.com. 2025.
4. Ismaeil, E.M.H. and Sobaih, A.E. "A Proposed Model for Variation Order Management in Construction Projects." Buildings, 14(3): 726. MDPI. March 2024. [Variation orders implicated in over 60% of construction disputes globally; combined designer and owner decisions identified as leading cause of excessive variation orders].
5. Propertyzone (Fannuel Mirirayi, reviewed by Dennis Nkomo). "How Much Does It Cost to Build a House in Zimbabwe in 2026? A Real Cost Breakdown From Foundation to Completion." propzone.co.zw. Updated September 2026. For ZIMSTAT CEMPI and BMPI material price data, 15% contingency guidance, material substitution risk.
6. ZIMSTAT. Civil Engineering Materials Price Index (USD). Monthly reports, base January 2023 = 100. Data to July 2026. Zimbabwe National Statistics Agency, 2026. — diesel +35% spike March 2026, cited via Propertyzone.
7. ZIMSTAT. Building Materials Price Index (USD). Quarterly reports, base December 2022 = 100. Data to December 2025. Zimbabwe National Statistics Agency, 2026. [Wall tiles +60%, roofing +22%, timber +18.5%, cited via Propertyzone].
8. Formulux. "Rising Costs and Material Overruns in Zimbabwe's Construction Sector: What Diaspora Builders Need to Know." formulux.co.uk. Accessed September 2026. For cement quantity overrun example (1,000 bags budgeted, 1,200 required due to foundation depth changes), and description of Zimbabwe material price volatility.
9. ResearchGate (Academia.edu access). "Cost Overrun Factors in Construction Industry: A Case of Zimbabwe." Originally published in proceedings, 2019.