Building projects are late more often than they're on time. This isn't a secret. Ask anyone who's managed a domestic extension what happened to their programme and you'll hear a consistent story: the project took longer than quoted, for a range of reasons, some legitimate and some not. Understanding why delays happen, who bears responsibility for them, and what you can do about them saves you both money and a great deal of stress.
Types of Delay
Delays in construction fall into two broad categories: those caused by the contractor and those caused by factors outside anyone's control (or by the client). This distinction matters because it affects who bears the cost and time consequences.
Contractor-caused delays include: insufficient labour on site, poor sequencing of trades, late ordering of materials, quality failures requiring rework, and inadequate planning of the programme. These are the contractor's responsibility. Under a standard contract, the contractor remains liable for completion by the agreed date regardless of these problems, and delay damages may apply if completion is late.
Relevant events (to use the JCT terminology) are circumstances that entitle the contractor to an extension of time: exceptionally adverse weather, statutory undertakers' delays, discovery of unforeseen ground conditions, variations instructed by the client, late information from the client, and certain force majeure events. A contractor who wants to extend the programme due to one of these events must notify the client promptly and in writing, setting out the cause and the extension claimed.
Client-caused delays occur when you are late providing information, late making decisions, late making stage payments, or when you instruct variations that disrupt the programme. These delays are your responsibility. In a standard JCT contract, you may also owe the contractor additional costs (loss and expense) if your delay has caused them financial harm.
The Importance of Having a Completion Date
If your contract doesn't specify a completion date, you have very limited remedies for delay. A contract that says only "the work will take approximately 16 weeks" is much weaker than one that says "practical completion shall be achieved by [date]." The first is aspirational. The second is contractual.
Insist on a specific completion date in the contract. The JCT Homeowner Contract includes a completion date field precisely because this is important. If the contractor won't commit to a date, that reluctance is itself a signal about how they intend to manage the programme.
Liquidated Damages
A liquidated damages (LD) clause allows you to deduct a specified daily or weekly amount from the contract sum if the contractor fails to complete by the agreed date. The amount doesn't have to be your actual loss: it's an agreed pre-estimate of the damages you'll suffer for late completion.
For residential projects, LD rates might be set at £150-£500 per week, reflecting things like alternative accommodation costs, storage costs, or general inconvenience. The rate must be a genuine pre-estimate, not a penalty. Courts will not enforce punitive LD clauses.
Many residential contractors resist LD clauses. Some refuse them entirely. Their resistance doesn't mean you can't ask, and for larger projects (extensions over £80,000, loft conversions with significant temporary displacement) they're worth pushing for.
If you instruct variations, the contractor is entitled to claim an extension to the completion date. This is fair. But make sure extensions are properly quantified and agreed in writing. A pattern of "oh that change will just add a couple of days" without formal extension of time agreements can accumulate to weeks of additional programme without any clear record of why.
When the Project is Slipping
The time to address a delay is when it first appears, not six weeks later when the programme has drifted substantially. At the first weekly site meeting where progress is behind plan, ask directly: how do you intend to recover this? What's the revised programme?
A contractor who has a plan for recovery and can articulate it is managing the situation professionally. A contractor who gives you reassurances but no specifics ("we'll get it back, don't worry") is not managing it at all. Press for specifics: what trades on what days, what needs to happen for the recovery to be achievable?
If recovery doesn't materialise, write formally to the contractor noting the slippage, the revised projected completion date, and your expectation that they will accelerate works to achieve the contractual completion date. This letter starts the formal record trail if you later need to pursue liquidated damages or dispute the delay.
Material Supply Delays
Since 2020-21, material supply chain disruptions have added a new category of delay that affected many projects. Lead times for structural steel, timber, glazing, and specialist items stretched significantly. While supply chains have largely stabilised, lead times for certain products remain longer than pre-pandemic norms.
The right approach is for the contractor to identify long-lead items at tender stage and order them early. If a builder doesn't mention lead times in their programme, ask explicitly: what items need to be ordered now to avoid delays? Kitchen units, windows, structural steel, bespoke joinery, specialist mechanical equipment, tiles from abroad: any of these can have lead times of 8-16 weeks.
Late material ordering by the contractor is a contractor-caused delay. If they failed to order materials that were known to have long lead times, they cannot claim that as a relevant event entitling them to an extension.
Pushing for Practical Completion
Practical completion is the point at which the works are substantially complete: the building is ready for occupation, even if minor snagging items remain. It's a contractual milestone that triggers several important events: the defect liability period starts, the client takes possession, the majority of the contract sum becomes payable (the retention is released over time), and risk transfers.
Some contractors use snagging items as an excuse to defer practical completion and avoid the defect liability clock starting. This is not correct. A long snagging list does not prevent practical completion: the works can be practically complete with minor defects to follow. Equally, some clients refuse to agree practical completion hoping to withhold payment. That's also incorrect.
When you believe the works are substantially complete, raise it with the contractor and, if you have an architect or contract administrator, with them. Get it formalised. Start the defect liability period, retain the appropriate retention, and move forward.