Retention in Construction: A QS Guide to How It Works

Retention in construction is a percentage of every payment certificate that the employer or main contractor holds back until the works are complete and any defects have been made good. For quantity surveyors, understanding retention construction clauses is not optional: it affects cash flow forecasting, subcontractor accounts, final account negotiations, and increasingly, the risk of insolvency further down the supply chain.

This guide sets out exactly how retention works in practice: how it is calculated, how JCT and NEC contracts differ in release mechanics, what typical defects periods look like, and how disputes over retention tend to arise. It also covers the biggest development in this area for a generation — the UK government's confirmed plan to ban cash retentions, announced in March 2026.

Whether you're preparing interim valuations, chasing a final retention release for a subcontractor, or advising a client on contract drafting, the detail matters. Small differences in wording between a JCT rectification period and an NEC4 Option X16 defects date can mean months of difference in when cash actually lands.

We'll walk through the mechanics with real percentages and timelines, then look at what's changing and what QS professionals should be doing now to prepare.

How Retention Works in a Construction Contract

The purpose of retention

Retention exists to give the paying party leverage. Construction work is certified and paid for in stages, often before it can be fully tested or inspected — a roof covering, for example, cannot be properly assessed until it has weathered a winter. By holding back a slice of every payment, the employer or main contractor keeps a financial incentive in place for the contractor to return and fix anything that goes wrong, rather than walking away once the bulk of the contract sum has been paid.

How the deduction is applied

Retention is deducted at source from each interim certificate. If a subcontractor's valuation for the month is £100,000 and the retention rate is 5%, £5,000 is withheld, and the subcontractor is paid £95,000. This accumulates across the contract, so by practical completion a contractor may have several thousand pounds — or on larger packages, six figures — sitting in retained cash that belongs to them but which they cannot yet draw down.

Where retention sits legally

Retention money is not automatically held in trust in England and Wales unless the contract says so (Scotland and some public sector frameworks have moved toward trust arrangements). This matters enormously when a main contractor or employer becomes insolvent — retention held as ordinary cash simply becomes an unsecured debt in the insolvency, which is one of the central arguments behind the government's push to reform the system.

How Much Retention Is Withheld and When Is It Released?

Retention percentages and release triggers vary by contract form. The table below summarises typical practice across the main UK construction contracts used by QS teams.

The Retention Release Timeline in Practice

The clearest way to picture retention is as a curve: it builds steadily through the construction phase, drops sharply at practical completion, and reaches zero once the defects period closes and the final certificate is issued. The graphic below shows how much of the retention pot is typically still held at each stage of a JCT-style project.

Under JCT contracts, the first moiety (half) of retention is released when the Practical Completion Certificate is issued, and the second moiety follows the Certificate of Making Good Defects, once the contract administrator confirms that defects notified during the Rectification Period have been remedied. That Rectification Period is commonly 12 months but is set out in the Contract Particulars, so always check the specific project figure rather than assuming a default.

NEC4 handles retention differently and, critically, it is not automatic. Retention only applies if Option X16 has been selected and completed in the contract data — a surprising number of NEC contracts run with no retention at all because X16 was never incorporated. Where X16 does apply, the first half of the retention is released at Completion and the second half at the Defects Date, typically 52 weeks after Completion. Unlike JCT, release of the second half under NEC4 is not conditional on every defect being corrected; it is tied purely to reaching the Defects Date, though the employer can still recover the cost of outstanding defects separately.

Common Retention Disputes and Risks

• Late or non-payment at practical completion — the paying party simply doesn't process the first moiety on time, often because no one has diarised the certificate date.
• Disputed defects — the contract administrator withholds the second moiety citing snagging items the contractor disputes are within scope or workmanship.
• Insolvency in the supply chain — retention held as unsecured cash is lost or heavily discounted when a main contractor or developer enters administration.
• Ambiguous drafting — bespoke amendments to standard JCT or NEC clauses create uncertainty over exactly when and how much should be released.
• Forgotten retention — on long-running frameworks, small retention balances against closed accounts are simply never chased and go unpaid for years.

Adjudication remains the fastest route to recovering unpaid retention, and the Construction Act gives an implied right to interim payment even where a contract is silent on the point. Retention disputes are rarely about whether the money is owed — they're almost always about timing, evidence of defects rectification, and whether a valid pay less notice was served before the deduction was made.

For QS teams, the practical defence is administrative discipline: log every retention deduction against the specific certificate, calendar every release trigger the moment practical completion or Completion is certified, and keep a running retention schedule as part of the CVR so nothing sits forgotten in a closed account.

The 2026 Retention Ban: What's Changing

On 24 March 2026, the UK government confirmed the toughest crackdown on late and deferred construction payments in more than 25 years, and the headline measure is a ban on withholding cash retentions under construction contracts. Rather than the softer option of ring-fencing retentions in third-party bank accounts, ministers opted for an outright ban, though the detail of implementation — including timing and any transitional provisions — is still subject to further consultation.

The reform sits alongside two other significant changes: a 60-day maximum payment term for large firms paying smaller supply chain businesses, and mandatory statutory interest of 8% above the Bank of England base rate on late commercial payments. Government figures cited in the announcement put the cost of late and deferred payment to the UK economy at roughly £11 billion a year, with an estimated 38 businesses closing daily as a direct consequence.

The Building Safety Act 2022 has added pressure in the other direction. As accountability for the safety and quality of completed buildings has increased, many employers have leaned on retention even more heavily as a defects safeguard — which is exactly the tension the retention ban now has to resolve. Expect the consultation to focus on alternative security mechanisms such as performance bonds, parent company guarantees, or retention bonds, since employers will still want some form of leverage over defects once cash retention disappears.

For QS professionals, the direction of travel is clear even before the legislation lands: build alternative security options into your contract advice now, understand how retention bonds and guarantees compare in cost and administration to cash retention, and flag to clients that retention clauses drafted today may need revisiting once the ban takes effect.

Managing Retention as a QS: Practical Tips

• Maintain a live retention schedule alongside your CVR, showing amount withheld, release trigger, and target date for every subcontract package.
• Confirm at contract award whether NEC4 Option X16 has actually been incorporated — don't assume retention applies by default.
• Diarise practical completion and defects-period end dates the moment they're certified, not when a subcontractor chases you.
• Check whether the contract requires retention to be held in a separate trust account, particularly on Scottish or public sector projects.
• Where retention bonds are used instead of cash, verify the bond wording matches the release triggers in the building contract — mismatches cause disputes.
• Issue a valid pay less notice in good time if you intend to withhold retention beyond the certified sum, to avoid a smash-and-grab adjudication.

Frequently Asked Questions

What is retention in a construction contract?

Retention is a percentage — typically 3% to 5% — of each interim payment that the employer or main contractor withholds from the contractor as security against defective work. It is released in two stages, generally half at practical completion and half once the defects period ends.

How much retention is normally withheld in UK construction?

5% is the most common rate under JCT contracts, sometimes reduced to 3% on larger or lower-risk projects. NEC4 typically sits in the same 3-5% range, but only applies at all if Option X16 has been included in the contract data.

When is retention released under a JCT contract?

The first half (moiety) is released when the Practical Completion Certificate is issued. The second half follows the Certificate of Making Good Defects, once defects notified during the Rectification Period — usually 12 months — have been remedied.

Does NEC4 always include retention?

No. NEC4 retention only applies if Option X16 is expressly selected and completed in the contract data. If X16 isn't included, there is no contractual retention mechanism at all, regardless of what a subcontract order might state.

Is retention being banned in the UK?

The government confirmed on 24 March 2026 that it intends to ban the withholding of cash retentions under construction contracts, as part of a wider payment reform package. Implementation timing and transitional detail remain subject to consultation, so retention clauses still apply on current contracts.

What happens to retention if a contractor becomes insolvent?

Unless the contract specifically requires retention to be held in trust, retained cash is not automatically protected. If the paying party becomes insolvent, retention owed to the contractor typically ranks as an unsecured debt in the administration, which is a key driver behind the retention ban.

What's the difference between retention and a defects liability period?

The defects liability period (or Rectification Period under JCT) is the timeframe during which a contractor is obliged to return and fix defects. Retention is the financial mechanism — the money withheld — that gives that obligation teeth. The two run in parallel but are not the same thing.

Final Thoughts

Retention has been a fixture of UK construction payment practice for decades, and for good reason — it gives employers real leverage over defects on a product that can't always be fully tested before handover. But the system has also tied up significant cash for contractors and subcontractors, often for far longer than the drafting intended, and left retained sums exposed whenever a paying party runs into financial trouble.

With the government's ban confirmed in principle, the mechanics covered in this guide — JCT moieties, NEC4 Option X16, defects periods, and the discipline of chasing release on time — remain essential knowledge for any QS working on live contracts today. The next few years will likely see retention bonds and other security mechanisms take a bigger role, and QS professionals who understand both the old system and where it's heading will be best placed to advise clients through the transition.

Want the full picture? Want the full picture on construction payments?

For more on how payment mechanisms affect QS practice, see our guides to Interim Valuations and Payment Certificates, JCT vs NEC Contracts, and Cash Flow Management for Quantity Surveyors on Surveyor Success.