How to Structure Stage Payments for a Home Extension

A home extension is one of the largest financial commitments you will make outside of buying the property itself. A single-storey rear extension in the South East of England typically costs between £40,000 and £80,000. A double-storey extension can exceed £120,000. At these figures, the way you structure payments is not a detail – it is the primary mechanism controlling your financial risk.

Stage payments – also called milestone payments or progress payments – break the total project cost into instalments tied to completed phases of work. The builder gets paid as they deliver. You only release money when you can see the work is done. It is the fairest and most practical approach to funding a home extension.

Why a single lump sum is never the right approach

Paying a builder the full contract value before work starts, or even after the first week, removes all financial incentive for timely completion. It also exposes you to the maximum possible loss if something goes wrong.

This is not hypothetical. Homeowners who pay large sums upfront consistently report the most severe outcomes when projects go wrong – because there is nothing left to withhold, no leverage to negotiate, and no fund to pay a replacement builder if the original one defaults.

Even paying in two large instalments (half at start, half at completion) is poorly structured. The builder has half your money before any meaningful work is done, and you have no payment mechanism to address problems that emerge during the build.

Stage payments solve this by aligning your outgoings with the builder's deliverables. Each payment corresponds to a verifiable phase of construction.

A typical stage payment schedule for a home extension

While every project is different, a five-to-seven stage payment schedule covers most single-storey and double-storey extensions. Here is a framework you can adapt:

Stage 1 – Mobilisation and groundworks (10–15% of contract value). This covers site setup, demolition of existing structures, excavation, and preparation for foundations. Payment is due when the ground is excavated and ready for foundations to be poured.

Stage 2 – Foundations and drainage (15–20%). Concrete foundations, below-ground drainage connections, and damp-proof course. Payment is due when foundations are complete and signed off by Building Control.

Stage 3 – Superstructure to roof level (20–25%). Brickwork, blockwork, steels, lintels, and roof structure including felt, battens, and tiles or slates. This is the most visually dramatic stage and represents the largest single spend. Payment is due when the structure is watertight.

Stage 4 – First fix (15–20%). Internal stud walls, first-fix plumbing, first-fix electrics, insulation, and any structural carpentry. Payment is due when all first-fix elements are installed and ready for plastering.

Stage 5 – Second fix and finishes (15–20%). Plastering, second-fix electrics and plumbing, kitchen and bathroom fitting, flooring, decoration, and external works. Payment is due when the property is substantially complete and liveable.

Stage 6 – Snagging and final completion (5–10%). A retention amount held back until all snagging items are resolved and the Building Control completion certificate is issued. This final payment is your strongest lever to ensure everything is finished properly.

The percentages above are indicative. Adjust them based on your specific project – a loft conversion has a different cost profile to a ground-floor extension, and a project involving significant structural work (underpinning, party wall, steel frame) will weight more heavily toward the early stages.

How to define what "complete" means for each stage

The most common source of disputes in stage-payment arrangements is ambiguity about what constitutes completion. If your contract says "pay 20% when the roof is done" but does not define whether that means tiled, felted, or just structurally complete, you are inviting an argument.

For each stage, your contract should specify the deliverables in plain language. What work will be done, what materials will be used, and what sign-off is required before payment is released. Where Building Control inspections are involved (foundations, drainage, structural elements), tie the payment to a passed inspection.

Photograph each completed stage before releasing payment. This creates a record that protects both parties – the builder has evidence of what was delivered, and you have evidence of what you paid for.

The role of a retention in protecting your interests

A retention is a percentage of the total contract value (typically 5–10%) that is held back until all work is complete, snagging is resolved, and any defects identified within an agreed period have been rectified.

Retentions are standard practice in commercial construction, but many residential homeowners either do not know about them or feel uncomfortable proposing one. You should not. Any professional builder expects a retention to be discussed, and most will agree to one without resistance.

The retention gives you a financial lever to ensure the builder returns to fix any issues after practical completion. Without it, you have no incentive mechanism once the final payment is made – and getting a builder to return for snagging after they have been paid in full is one of the most common complaints in residential construction.

Why escrow makes stage payments work better

Stage payments on their own are a significant improvement over lump-sum or two-instalment structures. But they still rely on direct transfers – which means the builder has your money in their account after each stage, and you have no protection if there is a dispute about what was delivered.

Milestone-based escrow services such as Renopay add a layer of protection by holding the funds in an FCA-regulated account between you and the builder. The builder can see the funds are committed (proof of funds), but the money only releases when the milestone is approved. If there is a disagreement about whether a stage is complete, the funds stay in escrow while it is resolved.

This is particularly valuable for self-managing homeowners who do not have a project manager or quantity surveyor providing independent oversight. If you are running your own renovation, escrow provides the structural protection that a professional intermediary would normally offer.

Use the Renopay Quote Analyser to check whether the stage-payment breakdown in your builder's quote is reasonable before you sign the contract.

Getting your builder to agree to stage payments

Most professional builders are comfortable with stage payments – they are used to them on larger projects and increasingly expect them from informed homeowners. If a builder objects to a reasonable stage-payment schedule, ask why. There may be a legitimate reason (for example, a bespoke kitchen that must be ordered and paid for in advance), or it may indicate that their cash flow relies on holding your money before doing the work.

Present the stage-payment schedule as part of the contract discussion, not as an afterthought. Frame it as something that protects both parties: you get confidence that payments match progress, and they get certainty about when they will be paid at each stage. Platforms like Renopay make this conversation easier by providing a structured framework that neither party needs to build from scratch.