"While you're here, could you look at that other window too?" It's the most common sentence in installation. Without a clear change order process, it ends in a payment dispute. With one, it becomes a seamless revenue stream. Here's the framework.

Why change orders cause disputes

The problem is simple: no signed document exists for the extra work. The customer assumed it was "thrown in"; you assumed it was extra. That conversation after the fact is uncomfortable — and legally you're on the weaker side without written confirmation.

Under English and EU law, verbal contracts are valid but carry a heavy proof burden. If the customer disputes the charge, you need to prove the agreement — and "I told them on-site" rarely holds.

The 3-step change order process

Step 1: Recognize it immediately

The moment a customer asks for something outside the original scope, say immediately: "I can do that — it's additional work so I'll send you a quick scope and price before we start." Never promise. Never do the work and invoice later without prior confirmation.

Step 2: Written confirmation — even simple

A change order doesn't need to be formal. Two lines via WhatsApp or email are legally usable evidence:

"Additional bathroom window (50×80cm, double glazed, same profile): £580 including fitting, plus VAT. Confirmed?"

Customer replies "Yes" or "Confirmed." Keep that thread. That's your written agreement.

Better: send a micro-quote PDF via a customer portal with a digital sign button. Customer signs, you have a date-stamped signature. Takes no longer than sending a WhatsApp once the workflow is set up.

Step 3: Invoice that references the original job

If you combine change orders into the final invoice, show them as separate line items: "Change order: additional bathroom window." If a separate invoice, reference the original job number. Clean paper trail, no surprises.

Common mistakes and how to avoid them

Mistake: "I'll mention it later" — customers who don't expect extra costs always dispute them. Always confirm before starting.
Mistake: Verbal agreement on-site with the fitter — the fitter is not a contracting party. Any scope changes need to go through you or the project manager, in writing.
Mistake: Surprising the customer on the final invoice — for longer projects, send an interim change order summary so the final invoice is never a shock.
Mistake: Not applying correct VAT to change orders — if the change order is renovation labour on an older property, the reduced VAT rate applies to the labour portion. Treat it the same as the original quote.

Template: change order clause for your quotes

Add this standard clause to all quotes so the rules are clear from the start:

"Work not described in this quote but requested during the project will be agreed in writing as a change order before execution and invoiced separately. The client will receive a scope and price before any additional work commences."

How Kozynofferte handles change orders

From the dashboard, send a supplementary quote to an existing customer in two steps: select the customer, add the extra window line, send via the customer portal. The customer signs digitally and the change order signature is logged with timestamp.{" "} Try free for 14 days.

Specific change order situation you're dealing with?{" "} Email me directly.

— Richard Bajnath, Kozynofferte

How to handle change orders in window installation without disputes