GRR redesigned signage, fixtures, graphics and store layout for the homeware stores.
The Birmingham-based design agency paid particular attention to use two- and three-dimensional designs to attract customer attention.
Jackie Passamore, Aga marketing manager, said: "The appointment of Green Room Retail as our design agency will ensure that customers are delivered a first-class shopping environment that reflects the quality of our brand's proposition."
Richard Ash, GRR founder and managing director, called the account a significant win and said he was looking forward to "driving the sales of their premium products" through the stores.
Aga makes traditional cast-iron cookers and Fired Earth sells upmarket homeware such as tiles, pain, bathrooms and home furnishings.
GRR was launched in 2001 and boasts clients including Vodafone, CAT and Umbro.
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