Construction and building services· Explainer
Bonds, Guarantees, Retention and Z Clauses — Read These Before You Price
Two contractors can price the same drawings and arrive at very different numbers, because the real cost of a construction contract sits partly in its commercial terms. These are the ones to read before you price, not after you win.
A construction tender is usually priced from the drawings, the specification and the bill or activity schedule. The commercial terms that shape what the job is really worth tend to be read later, sometimes much later.
They live in places that are easy to skim: the contract data in an NEC contract, the contract particulars in a JCT one, and a schedule of amendments that may run to dozens of pages. This piece walks through the terms that most often change a price or a bid decision. It is a reading guide, not legal advice.
Where the terms live#
In NEC4's Engineering and Construction Contract, the client chooses a main option, from Option A (priced contract with activity schedule) to Option F (management contract), and then a set of secondary options, numbered X1 onwards, which add or change particular terms. Anything the standard options don't cover is added as additional conditions of contract, known as Z clauses. The choices, and the figures that go with them, are set out in the contract data.
JCT contracts carry many of the same decisions in the contract particulars, such as the retention percentage and the liquidated damages rate. Bond and guarantee requirements are set out in the contract documents, and bespoke changes usually appear as a schedule of amendments.
Either way, the rule is the same: find the contract data or particulars, and the amendments, before you start pricing.
The main option has the biggest effect of all on risk. The construction tender pack explains the six NEC4 main options. This piece covers the terms layered on top.
Performance bonds (NEC Option X13)#
A performance bond is a guarantee from a third party, usually a bank, insurer or surety, that pays out to the client if you default. Under NEC4, Option X13 requires the contractor to obtain a performance bond in the client's favour.
At tender stage, a bond requirement raises three practical questions:
- Can you get one? The bond provider assesses your finances. A requirement you can't satisfy is a knockout, however good the rest of the bid.
- What does it cost? A premium or fee, and it may use up facility capacity you need for other work. Price it in.
- How long does it take? It runs on the provider's timetable, not yours. Ask early. It is one of the documents that holds up tender submissions.
Check the required amount, form and timing in the contract data, and whether the bond is needed at award or only before work starts.
Parent company guarantees (NEC Option X4)#
If the client wants the financial strength of your group behind the contract, it may ask for a guarantee from your parent or ultimate holding company. In NEC4 this is Option X4, the ultimate holding company guarantee.
Two things to know:
- It needs the parent's agreement, on the parent's timetable. That usually means its board and its lawyers.
- It can be part of how you qualify. Under the Procurement Act 2023, you can rely on another company meeting a condition of participation, such as financial standing, if the buyer is satisfied you will enter legally binding arrangements under which that company will guarantee your performance of the contract. The company you rely on then counts as an associated person in the PSQ.
Retention (NEC Option X16)#
Retention lets the client hold back a proportion of each payment as security against defects. Under NEC4 Option X16, part of the retention is repaid on completion of the whole of the works, and the balance when the defects certificate is issued.
For a contractor, retention is a cash flow cost on every valuation, often for a long time. Check the percentage, any limit on the total retained, and when it is released.
The proposed ban#
The government has said it will stop the practice. Its announcement of the Bill on 19 May 2026 included “a ban on the practice of withholding retention payments under construction contracts”. The Bill, now called the Commercial Payments Bill, completed its stages in the House of Lords in October 2026.
As of early November 2026, it is not yet law. Contracts tendered now may still include retention, and existing contracts are affected only once the legislation is passed and in force, on whatever terms and timetable it finally sets. Until then, read the retention clause as written, and price its cash flow cost.
Delay damages and limits of liability (NEC Options X7 and X18)#
Delay damages (liquidated damages in JCT terms) are a pre-agreed amount payable for each day or week of late completion. Under NEC4 Option X7 they are deducted from what you are paid. Check the rate against your margin: a high rate on a tight programme can wipe out the profit on a job.
Limitation of liability (Option X18) sets caps on your liability, such as an overall cap and lower caps for some kinds of loss. Check whether the contract caps your liability at all, and at what level. Ask your insurer whether your cover matches.
Z clauses and amendments#
Z clauses are where bespoke risk lives. They can change payment periods, add time limits for notifying claims, shift design responsibility, widen indemnities, or rewrite how compensation events work. One amendment can quietly move a risk that the standard form would have left with the client.
Three habits help:
- Read every one before pricing. Not after the quality questions are written.
- Compare against the standard form. The question for each one is what it changes, not what it says.
- Ask about anything unclear or disproportionate. A clarification question that quotes the clause and asks how it is intended to work costs nothing. Discovering its meaning after award can cost a great deal.
If you are a sub-contractor#
The same terms usually flow down. A main contractor with retention, delay damages and a bond in its contract will often pass versions of them on to its supply chain.
One protection flows down the other way. For public contracts awarded under the Procurement Act 2023, the government's PSQ template asks suppliers to confirm they have systems in place to include, as a minimum, 30-day payment terms in all of their supply chain contracts, and to require those terms to be passed down the chain.
A five-minute pre-pricing check#
| Term | Where to look | The question to answer before pricing |
|---|---|---|
| Main option | Contract data, part one | How are we paid, and who carries cost risk? |
| Performance bond | X13 / particulars | Can we obtain it, at what cost, and by when? |
| Parent company guarantee | X4 / particulars | Will our parent sign it, and how long will that take? |
| Retention | X16 / particulars | What percentage, what limit, and when is it released? |
| Delay damages | X7 / particulars | Does the rate fit our margin and programme? |
| Liability cap | X18 / particulars | Is liability capped at all, and does our insurance match? |
| Z clauses and amendments | Additional conditions / schedule | What does each one change from the standard form? |
None of these terms is unusual. All of them are easy to price correctly once you have read them, and easy to price wrongly if you have not.