Direct Answer
A design change is any modification to the approved engineering drawings that changes what you fabricate — additional members, revised connections, section upgrades, or scope additions. You get compensated through a Change Order (CO) or Variation Order submitted to the GC, supported by documentation showing what changed, when it changed, and what it costs to implement.
Full Explanation
The chain of documentation for a change order: (1) Design change identified through RFI response, revised drawing, or addendum. (2) Fabricator notifies GC in writing of the change and its cost impact. (3) A Change Order Proposal (COP) is submitted with scope description, unit costs, labor hours, and schedule impact. (4) The GC and owner negotiate and execute a Change Order. (5) The change is incorporated into the contract. Failing to document design changes before absorbing them is the primary way fabricators lose money on projects.
What This Means for Your Shop
Never perform work you believe is outside your original contract scope without a written Change Order or at minimum a Change Directive (instruction to proceed while CO is negotiated). Work without documentation is work you typically cannot recover.
Common Mistakes
Proceeding with design changes under verbal assurances that 'you'll be taken care of.' The GC's project manager who verbally approved your change may leave the project, and the replacement will deny knowledge of any agreement.
Recommended Process
When a design change is identified, send a written notice to the GC within 48 hours identifying the change, referencing the document that changed it, and stating that a Change Order Proposal is forthcoming. Follow with the COP within 5 business days.