A chartered PQS practice in the North West takes on the cost-consultancy role for a privately-funded thirty-two unit residential scheme. The Stage 3 cost plan is issued at £8.2m; a Stage 4 update revises to £8.6m after value-engineering inputs from the design team. Procurement runs on a two-stage basis with a contractor brought in early; a JCT Design and Build 2024 contract is let at £9.05m once contractor's design portion items have been priced. Practical completion arrives roughly on programme. The final account, once variations and loss-and-expense are agreed, settles at £10.4m — £1.35m above the Stage 4 cost plan. The developer's funders have already drawn additional finance to cover the overrun and the developer's accountants have written into the developer's own audit file that the project lost money. Two months later the QS practice receives a letter of claim alleging negligent cost estimation, inadequate contingency provision, and a failure to advise the developer of the inflationary risk inherent in single-source procurement of a fixed-price contract.
That letter is the textbook QS PI claim. The pleaded loss may be £1.35m; the defence cost will be a six-figure sum before the matter is anywhere near a trial; and the practice's renewal — already three months away — will be priced in light of the notification. This article looks at how cost-overrun PI claims against quantity surveyors actually arise, what RICS expects the firm to have done at the cost-planning stage, how a "should have known" allegation is defended, and what the indicative settlement and defence-cost ranges look like in 2026. It sits within the broader quantity surveyors PI pillar guide and is the sibling article to our construction expert witness PI cluster.
How a cost overrun becomes a PI claim
A cost overrun in itself is not a PI claim. Construction projects overrun their budgets for a wide range of reasons that have nothing to do with QS negligence — design development, employer-driven scope change, market inflation, contractor pricing failure, ground conditions, weather, supply-chain disruption, late information from third parties, regulatory changes during the project. For a QS to be liable, the developer must show that the QS breached the duty of reasonable skill and care owed under the appointment, that the breach caused identifiable loss, and that the loss flows naturally from the breach in a way English contract and tort law will recognise.
The developer's case typically runs along one or more of the following lines. First, that the cost plan itself was negligently prepared — built on incorrect rates, on inadequate measurement of the design then available, on insufficient contingency, or on assumptions that no competent QS would have made. Second, that the cost plan was reasonable at issue but should have been updated as design developed and was not — the "frozen cost plan" allegation. Third, that the QS failed to advise on inflationary risk, contingency adequacy, or the contractual mechanism by which cost certainty was being procured. Fourth, that the QS failed to spot or advise on contractor pricing inconsistencies during tender analysis. Fifth, that during contract administration the QS over-certified, mis-valued variations, or under-resisted contractor loss-and-expense claims, causing the employer to pay more than it should have.
Each of these has a recognised defence and each has a recognised line of attack. The dispute is rarely about whether the overrun happened — the developer has the final account to prove it did — but about whether it was caused by anything the QS did or failed to do.
The RICS APC duty of care
A chartered quantity surveyor is held to the standard of a reasonably competent practitioner in the QS & Construction pathway of the RICS APC. That standard is not perfection; it is not the standard of the most cautious or most experienced QS in the market; it is what the reasonable practitioner in equivalent practice would have done in the same circumstances with the same information.
In practical terms the standard imports the RICS guidance notes and practice statements current at the date of the work — the New Rules of Measurement (NRM 1 for order-of-cost estimating and cost planning, NRM 2 for detailed measurement, NRM 3 for life-cycle cost planning), the RICS guidance on contingency allowances, the RICS guidance on cost reporting, and the underlying principle that a cost plan is a snapshot of reasonable expectation at a defined point with defined assumptions, not a guarantee.
The "should have known" allegation — the developer's lawyers asserting that the QS should have foreseen a particular cost increase — is met principally with contemporaneous evidence. What did the QS actually know at the date of the cost plan? What did the design team's information actually show? What were the published market indices for the relevant trades? What did the QS document about the assumptions, the qualifications, the risk register, the contingency rationale? The strength of the defence almost always tracks the quality of the file. A well-documented cost plan with a clear risk register, explicit assumptions, a contingency rationale and a record of what the developer was told, is far harder to attack than a one-page summary issued by email.
Where measured-quantity errors meet tender-return pricing
A particular sub-pattern within cost-overrun claims arises where the QS prepared bills of quantities or pricing documents for tender, those documents contained measurement errors (undermeasured items, missing items, errors in description), the contractor priced what was on the page, and the omissions were corrected during the contract as variations at variation rates rather than tender rates. The employer pays more because the original tender was incomplete; the contractor argues the tender priced what was provided and the variations are properly chargeable.
The claim against the QS is that competent measurement would have produced a complete tender, the contractor would have priced the full scope at tender competition, and the employer would have paid the lower competitive figure rather than the higher variation figure. The defence runs on the standard of measurement reasonably achievable at the design stage the QS was working from, on the contractor's own pre-tender review obligations, on any qualifications stated in the tender documents, on the magnitude of the alleged omissions relative to the total contract sum, and on the chain of causation between any measurement defect and the actual additional cost.
Where the design provided to the QS was itself incomplete or inconsistent — drawings late, design information caveated, specifications evolving — the chain of responsibility is shared. The QS's defence is rarely that there were no measurement issues at all; more typically that the issues identified were within the tolerances reasonable for the design stage, were qualified appropriately in the tender documentation, or were caused by design information the QS could not have measured accurately from.
Value-engineering, contingency adequacy and the inflation point
Value-engineering exercises — running through the cost plan to identify savings, swap out specifications, defer scope — are a routine part of the QS's role in the months between Stage 2 and contract execution. They are also a recurring source of PI exposure. The complaint is that the value-engineered cost plan stripped contingency that the QS knew or should have known would be needed, deferred scope that was always going to come back as a variation, or specified savings that proved illusory once contractor pricing came in.
Contingency adequacy is the central technical question. RICS guidance does not prescribe a single contingency percentage; the figure depends on design maturity, procurement route, contractor selection, market conditions and project-specific risk. A 5% contingency on a Stage 4 cost plan for a complex residential scheme procured single-stage in a volatile market is a different proposition from 5% on a straightforward warehouse refurbishment in a stable market. The QS's defence on a contingency allegation typically rests on documented rationale — what the contingency was, why that level, what risks it was intended to cover, what risks it explicitly did not cover, and what was communicated to the client.
The inflation point — the allegation that the QS failed to advise the employer that letting a fixed-price contract in a volatile materials market would expose the project to under-pricing risk that the QS should have flagged — has become more common since 2022 as construction inflation surged. The defence sits on the contemporaneous information available, what the published market indices were showing, what advice the QS gave (and ideally documented) about procurement-route selection, and whether the employer chose to proceed on the route adopted with awareness of the risk.
The "should have known" allegation defended
Defending "should have known" turns on three things: contemporaneous documentation, expert evidence, and the framing of the standard of care.
Contemporaneous documentation — the working papers from the time of the cost plan or the contract administration decision — is the single most important defensive asset. Working papers include the cost-plan workings (rates used, sources for the rates, measurement back-up, contingency rationale, risk register, assumptions schedule), the file note of any client meeting at which the cost plan was issued, the issue cover letter or email, the response correspondence from the client, the published cost-plan iterations, and any subsequent advice given as design developed. A practice with clean contemporaneous papers can show what was known, what was assumed, what was qualified and what was communicated; a practice without them is reliant on witness recollection years later.
Expert evidence is where the practical fight is conducted in most cases that reach proceedings or formal mediation. Both sides instruct an independent quantity-surveying expert (typically a senior chartered QS in equivalent practice) to give evidence under CPR Part 35 on what a reasonably competent QS would have done in the circumstances. The expert evidence frames the standard of care for the tribunal; a well-respected defence expert with clean working papers to work from is the practice's principal asset in resisting the claim. (The QS as expert witness is the subject of our construction expert witness PI cluster.)
Framing the standard of care is the legal work. The standard is reasonableness, not perfection; what the reasonably competent practitioner would have done at the date in question with the information then available, not what hindsight identifies as the optimal course. The defence team's job is to keep the tribunal anchored to the standard, not to a counsel-of-perfection re-run of the project.
Settlement ranges and defence cost profile
Cost-overrun QS PI claims settle across a wide range. Working from anonymised industry patterns, claims pleaded in the low six-figures often settle for a fraction of the pleaded sum once causation, contributory factors and the claimant's evidential gaps are tested. Claims at the £1m-plus pleaded level settle with greater variability — some discount heavily after defence-expert evidence undermines the causation case; others settle near the pleaded figure where the documentation is thin and liability looks indefensible. The defence's leverage typically peaks once defence-expert evidence has been exchanged and the claimant's case has been tested in correspondence; many cases settle in mediation in the months after exchange of expert reports.
Defence costs on a contested cost-overrun matter typically run from low five-figures on a straightforward early-settled claim through to mid- to high-six-figures on a matter that reaches trial in the Technology and Construction Court. The defence-cost profile is one of the reasons RICS minimum-term limits are floors rather than targets — a practice with a £1m primary limit defending a £750k pleaded claim through to trial can find a substantial portion of the limit consumed by defence costs alone, leaving limited headroom for any settlement.
The practical implication for limit-setting is to scale the policy not just to the size of the projects taken on but to the cost of defending a contested matter on those projects, with headroom. The pillar guide on quantity surveyors PI covers limit-sizing in more detail.
Notification — the moment the policy attaches
Because QS PI is written on a claims-made basis, the moment a circumstance is notified to insurers is the moment the policy attaches to that matter. A circumstance is anything that the firm becomes aware of which might give rise to a claim — a complaint letter, an angry phone call after a final-account meeting, a developer's expression of dissatisfaction with the cost outturn, a draft letter of claim received from solicitors.
Late notification — failing to notify a circumstance when first aware, and then trying to notify after the claim has crystallised on a later policy year — is the single most common reason an otherwise-covered claim fails. The policy that should have responded was the one in force when the circumstance was first known; the later policy excludes circumstances known before its inception. The practice can find itself uninsured for a claim it would have been fully covered for, had it notified six months earlier.
The discipline is simple in principle and harder in practice: anything that could give rise to a claim is notified to insurers, in writing, through the broker, promptly. Notifying a circumstance is not an admission of liability; it is the procedural step that preserves the cover. RICS-compliant wordings cannot exclude notification rights for matters notified within the policy period and on time.
Working with the broker on a notified matter
Once a circumstance is notified, the broker's role shifts from placement to claims support. The broker liaises with the insurer's claims team, helps the practice respond to information requests, and ensures the panel solicitors instructed by the insurer have the documentation needed to run the defence. The practice retains a direct stake in the conduct of the defence — RICS-approved wordings include consent provisions on settlement and on selection of solicitors — but the day-to-day mechanics run through the insurer's claims function.
At renewal the matter will be disclosed to the renewing market. Apex's standard approach is to brief the market on the matter early in the renewal cycle, with a clear narrative of what happened, what the firm did about it, what the firm has changed (procedural improvements, sign-off changes, additional supervision on similar engagements) and the current status. A well-managed notification with a coherent response history is materially easier to place than one presented to the market in fragments.
What the firm can do now
The renewal-cycle items worth working through in advance of a cost-overrun-prone period are the following.
- Confirm the per-claim limit is sized to the worst-case exposure on the three largest live cost-consultancy engagements, with headroom for defence costs.
- Confirm the wording covers all the firm's activities — pure PQS, employer's agent, project management, expert witness — without sub-limits or exclusions that would bite on a cost-overrun matter.
- Confirm the retroactive date is continuous with prior cover; a reset retroactive date can strand exposure on old projects.
- Confirm the excess level is workable given the typical cost-overrun claim profile — too low an excess multiplies administrative friction on smaller matters, too high a one moves uninsured risk onto the firm.
- Confirm internal procedures for cost-plan issue, sign-off, assumption-recording and client communication are documented and followed — the file you build today is the file the defence runs on in three years.
- Confirm the firm's notification procedure is understood across the partnership — who notifies, through whom, on what trigger.
How Apex helps
Apex Insurance Brokers is an independent FCA-authorised insurance broker. We act as the firm's broker under the Financial Conduct Authority's Conduct of Business rules, representing the firm's interests in negotiation with the RICS-approved PI market. We work regularly with chartered QS practices on cost-consultancy, employer's-agent and project-management portfolios — placing renewal cover, advising on wording for specific appointments, handling notifications and supporting claims through to resolution.
The terms on which we act are set out in our Terms of Business, our handling of personal data in our Privacy notice, and the route for raising concerns about our service is on our Complaints page. The quantity surveyors sector page is the place to start a renewal conversation, or contact us directly. If you have a circumstance you have not yet notified, that conversation is the priority — late notification is the single most common reason a covered claim fails to be paid.
Frequently asked questions
My cost plan was issued two years ago and the project has overrun — am I liable?
Overrun does not equal liability. The developer must show that the QS breached the duty of reasonable skill and care, that the breach caused identifiable loss, and that the loss is recoverable in law. Cost plans are snapshots at defined dates on defined assumptions; many overruns are caused by post-cost-plan factors (scope change, market inflation, contractor pricing, ground conditions) for which the QS is not liable. The strength of the defence usually tracks the quality of the contemporaneous working papers and the assumptions recorded at issue.
Should I notify insurers if the developer is just grumbling about the final account?
Yes. Notification is procedural — it preserves the policy's response. A circumstance is anything that might give rise to a claim, including expressions of dissatisfaction that could harden into a letter of claim. Notifying is not an admission of liability and not a trigger for the excess or for premium loading on its own; failure to notify is the single most common reason an otherwise-covered claim fails. Where the position is unclear, take the broker's view before deciding.
How much will the defence cost?
It depends on the claim's complexity and how far it runs. Working from anonymised industry patterns, defence costs on a straightforward early-settled matter can sit in the low five-figures; a contested mid-six-figure pleaded claim through to mediation typically runs through high five-figures to low six-figures; a TCC trial on a £1m-plus pleaded claim can consume mid- to high-six-figures in defence cost alone. Most RICS-compliant wordings pay defence costs within the limit, which is why limit-headroom over expected loss is so important.
Will my renewal premium increase because of a notification?
A notification is not in itself an automatic premium loading. The renewing insurer will look at the matter, the practice's response, what has changed procedurally, and the broader claims history. A single notification on an otherwise-clean account, well-documented and properly handled, typically renews more sensibly than the same notification handled fragmentarily. Multiple notifications, or a pattern of similar matters, will affect terms. The broker's role at renewal is to present the matter coherently to the market.
Can the developer claim against me personally as a partner?
In a traditional partnership, partners are jointly and severally liable for the firm's liabilities, including PI claims — which is why most chartered QS practices have moved to LLP or limited company structures. Within an LLP or limited company the personal exposure of individual members or directors is generally limited to the entity's assets, save in cases of fraud, dishonesty or specific personal-duty allegations. The PI policy responds to the firm's liability; the broker can advise on whether the firm's structure and cover align with the partners' risk appetite.
What does run-off look like if I retire mid-claim?
A claim already notified to the working policy continues to be handled under that policy's terms regardless of subsequent retirement — the policy attached at notification. For unnotified back-catalogue work the firm needs to put run-off cover in place from the date of cessation; six years is the practical floor under RICS minimum terms, longer where deed appointments or residential work are in the portfolio. The pillar guide on quantity surveyors PI covers run-off in more detail.
Related guides
- Quantity surveyors PI insurance — UK guide 2026 (pillar)
- Construction expert witness PI
- Quantity surveyors sector page — speak to a broker
- All Apex PI sectors
About Apex Insurance Brokers
Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority, FCA firm reference 724952. Registered in England and Wales, Companies House 07014570. Trading address QCS, 53 Queen Charlotte Street, Bristol BS1 4HQ; registered office c/o Westcan, 5 Anglo Office Park, Bristol BS15 1NT. Email info@apexinsurancebrokers.co.uk, telephone 0117 325 0027. This article is general information about Professional Indemnity Insurance for UK quantity surveying practices and is not advice tailored to any individual firm's circumstances. Last reviewed: May 2026.
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