Introduction
Variations are an unavoidable part of construction. No matter how thoroughly a project has been planned and specified, changes will arise once work begins on site. A client may want to upgrade a kitchen specification. An architect may revise the structural details after opening up existing walls. Ground conditions may differ from what the survey suggested. In every case, the original scope of works shifts, and with it the cost, the programme and the risk profile of the project.
For builders and contractors, the way you manage variations can make the difference between a profitable project and a loss-making one. Handled well, variations are simply part of the job — documented, priced, agreed and built. Handled badly, they become a source of disputes, cash flow problems and damaged client relationships that can follow you long after the scaffolding comes down.
This guide sets out a practical approach to dealing with variations in construction. It covers what variations are, why they happen, how to price them accurately and why written records are essential. Whether you are running a domestic extension or managing a multi-trade commercial refurbishment, the principles are the same: control the process, protect your margin and keep the project moving.
What Is a Variation in Construction?
A variation is any change to the scope, specification, quality or quantity of the works described in the original contract or agreed estimate. It can involve adding new work, omitting work that was originally included, or altering the way existing work is to be carried out. Variations may be instructed by the client, the architect, the contract administrator or the employer’s agent, depending on the form of contract in use.
Under most standard construction contracts, there is a defined mechanism for issuing and valuing variations. The JCT suite, for example, allows the architect to issue instructions that constitute variations, and the contract sets out how those variations should be valued — typically using the rates in the contract bills or, where no applicable rate exists, at a fair valuation. NEC contracts handle changes through a compensation event process, which requires the contractor to submit quotations before the work is carried out.
In domestic and smaller commercial projects, the process is often less formal, but the principle remains the same. Any change to what was originally agreed needs to be documented, priced and confirmed before the work proceeds. Without this discipline, you risk carrying out additional work at your own cost with no guarantee of payment.
Key Point: A variation is not simply ‘extra work’. It is a contractual mechanism that entitles the contractor to additional payment and, where applicable, additional time. Treat every variation as a formal instruction, regardless of the size of the project.
Common Causes of Variations
Understanding why variations arise helps you anticipate them, price for the risk and manage client expectations from the outset. The most common causes include:
Client-driven changes. The most frequent source of variations on domestic projects. A client may decide to change the layout of a bathroom, add underfloor heating to a room that was not originally specified, or upgrade from standard to bespoke joinery. These changes are often driven by the client seeing the space take shape and wanting to refine their vision. While understandable, each change has a cost and time implication that must be captured.
Design development. On projects where the design was not fully resolved before work began, variations arise as the architect or designer finalises details. This is common on refurbishment projects where existing conditions are only fully understood once opening up works are complete. Structural engineers may revise their specifications, or the architect may alter room layouts in response to what is found on site.
Unforeseen site conditions. Poor ground, hidden asbestos, unexpected drainage routes, structural defects concealed by finishes — all of these are situations that can only be confirmed once work is under way. The original estimate cannot reasonably have allowed for conditions that were not known or foreseeable at the time of pricing.
Regulatory requirements. Building control or planning conditions may impose changes that were not anticipated in the original design. A Building Control Officer may require additional fire stopping, upgraded insulation or structural modifications that go beyond the original specification.
Errors or omissions in the original documents. Drawings and specifications are prepared by human beings, and mistakes happen. Missing items, conflicting dimensions, ambiguous specifications and incomplete schedules all give rise to variations when the true scope of the work becomes clear on site.
Tip: At the start of every project, review the drawings and specification carefully for gaps and ambiguities. Flagging potential issues early — in writing — puts you in a much stronger position if those issues later become variations.
How to Price Variations Accurately
Pricing variations accurately is one of the most important skills a builder or contractor can develop. Get it wrong, and you either absorb costs you should not be carrying or create friction with the client by submitting prices that appear inflated. The key is to use a consistent, transparent method that the client or their representative can follow and verify.
Use the original rates where applicable. If the variation involves work that is similar in character and executed under similar conditions to items already priced in the original estimate, use those rates as the starting point. This is the approach most contracts require, and it provides continuity and fairness for both parties. For example, if you priced brickwork at a certain rate per square metre in your original estimate, and the variation involves additional brickwork of the same specification, the original rate should apply.
Price new items from first principles. Where the variation involves work that has no equivalent in the original pricing, build up the rate from scratch. Calculate the labour hours required, apply your current labour rates, price the materials at today’s costs, add for plant and equipment, include waste and fixing allowances, and apply your overheads and profit. This first-principles approach produces a rate that is defensible and auditable.
Account for disruption and programme impact. A variation does not happen in isolation. It may require operatives to stop what they are doing, clear an area, wait for revised drawings, or re-sequence work that was already planned. These knock-on effects have a real cost, and your variation price should reflect them. Additional preliminaries — such as extended site management, welfare provision or scaffold hire — should be included where the variation extends the programme.
If you are not confident pricing variations from first principles, or if the project involves complex or high-value changes, consider using a professional estimating service to prepare or check your variation costings. An independent valuation adds credibility and can help avoid disputes at final account stage.
Key Point: Always submit your variation price before carrying out the work. Agreeing the cost in advance removes uncertainty and gives both you and the client a clear financial picture at every stage of the project.
The Importance of Written Variation Orders
If there is one piece of advice that every experienced contractor would give to those starting out, it is this: get everything in writing. Verbal instructions on site are common — a client asks you to move a door opening, a site manager tells you to change a material — but a verbal instruction is worth very little when it comes to agreeing payment three months later.
A written variation order should record the following as a minimum:
- A clear description of the varied work, including any changes to specification or quantity
- The reason for the variation and who instructed it
- The agreed price for the variation, or a statement that the price is to be agreed
- Any impact on the programme, including any extension of time claimed
- The date the instruction was issued and the date work is to commence
- Signatures or written confirmation from both parties
On projects governed by a formal construction contract, the variation process will be defined in the contract conditions. Follow that process to the letter. On less formal domestic projects, a simple confirmation email setting out the above details is sufficient. The important thing is that there is a written record that both parties can refer to.
Failing to document variations properly is one of the most common reasons contractors lose money on projects. When a dispute arises at the end of a job, the contractor who has a folder of signed variation orders and confirmation emails is in a far stronger position than one who relies on recollection and goodwill.
Impact on Programme and Budget
Every variation has the potential to affect both the project programme and the overall budget. Even seemingly minor changes can have cumulative effects that, if left unmanaged, push a project over time and over budget. Effective variation management means tracking these impacts in real time, not retrospectively.
Programme impact. A variation may require additional time to complete the new or altered work. It may also cause delay to subsequent trades if the varied work sits on the critical path. For example, a client instruction to change the structural opening sizes in a ground floor extension will delay brickwork, which delays the roof, which delays the internal trades. The programme impact of a single variation can cascade through the entire project if it is not managed proactively.
Where a variation entitles you to additional time under the contract, submit your extension of time claim promptly and with supporting detail. Do not wait until the end of the project to raise programme issues that arose months earlier. Contracts typically require notice to be given within a defined period, and failure to comply with the notice provisions can weaken or invalidate your claim.
Budget impact. Maintain a running variation account that records every instructed variation, its agreed or estimated value, and its status (priced, agreed, pending). Share this with the client or contract administrator at regular intervals — monthly as a minimum. A transparent variation account avoids the unpleasant surprise of a large final account adjustment and gives the client the information they need to manage their own budget.
Tip: Issue interim variation statements with each monthly valuation or payment application. This keeps the financial position clear and reduces the risk of disputes at final account stage. Our post-contract services can help you manage this process professionally.
Retention, Defects and Final Accounts
Variations do not end when the physical work is complete. They form a significant part of the final account process, and any unresolved variations will delay the agreement of the final account and, by extension, the release of retention monies.
The final account is the agreed total sum payable under the contract, adjusted for all variations, daywork, provisional sums, loss and expense, fluctuations and any other contractual adjustments. It is typically prepared after practical completion and should represent a fair and accurate reflection of the true cost of the project as built.
For builders working under JCT or similar contracts, the final account must be submitted within the timescale set out in the contract conditions. Late submission can result in the employer or contract administrator preparing the final account on your behalf, which is unlikely to be in your favour. Ensure that all variation documentation — instructions, prices, confirmation emails, daywork sheets — is collated and submitted as supporting evidence.
During the defects liability period (also known as the rectification period), the client may identify snagging items or defects. It is important to distinguish between genuine defects in your work and items that fall outside the original scope or specification. Work that was varied during the project should be assessed against the varied specification, not the original. Keeping thorough variation records protects you against unfair claims during this period.
Retention is typically released in two halves: the first half at practical completion and the second at the end of the defects period. Until the final account is agreed, the employer has no obligation to release the second half of retention. Unresolved variations are the single biggest cause of delayed final accounts in UK construction, which is why getting them right from the start is so important.
When to Seek QS Support
Many builders manage variations perfectly well on straightforward domestic projects. A simple confirmation email, a priced schedule and a willing client are often all that is needed. But as projects grow in complexity, value or contractual formality, the risks associated with variations increase significantly.
Consider engaging Quantity Surveying support when:
- The project is governed by a formal construction contract (JCT, NEC, or bespoke)
- Variations are frequent, high-value or technically complex
- There is a dispute or disagreement over the valuation of varied work
- The client has their own QS or cost consultant, and you need professional parity
- The final account involves substantial adjustments from the original contract sum
- You are dealing with loss and expense claims arising from variations or delays
A Quantity Surveyor can prepare variation valuations, submit interim applications, negotiate the final account and provide contractual advice on your entitlements. This is particularly valuable on commercial projects where the sums involved justify the cost of professional support.
At First4Estimating, our post-contract services are designed specifically to support builders and contractors through the financial and contractual aspects of a project. From variation management and interim valuations to final account preparation, we provide the QS expertise you need without the overhead of a full-time in-house surveyor.
If you are working on a project where variations are becoming difficult to manage, or if you want to put a proper process in place before your next contract, get in touch with our team. We can advise on the best approach for your specific situation and provide the level of support that fits your business.