This study explains how consumers form attitudes toward sustainable fashion line extensions by integrating Brand Extension Theory (BET) with the Theory of Planned Behavior (TPB). Using survey data from 786 consumers in Portugal and Turkey, with H&M Conscious Choice as the focal reference, we estimate a structural model in which environmental concern (EC) and brand image (BI) shape attitude (AT) both directly and indirectly through brand‑extension fit (BEF). Results show that EC has a positive but small direct association with AT, while its indirect effect via BEF is also significant, indicating that value‑based dispositions become consequential when the sustainability initiative is perceived as a credible and logical extension of the parent brand. BI exerts a substantive direct influence on AT and a statistically reliable but modest influence on BEF; BEF itself positively predicts AT and transmits part of EC’s and BI’s effects. The model explains R² ≈ 0.742 of BEF and R² ≈ 0.476 of AT, supporting the explanatory adequacy of the integrated framework. A multi‑group analysis suggests context‑sensitive differences between Portugal and Turkey, which we interpret in light of policy‑normalization (Portugal) versus crisis‑salience (Turkey); given partial measurement invariance, these contrasts are framed as cautious tendencies. Overall, findings underscore fit as the mechanism through which consumers synthesize values and brand heuristics, clarifying why strong brand image alone does not guarantee perceived sustainability congruence in fast‑fashion contexts.