ABSTRACT This study investigates the heritage language development of Turkish-English bilingual children in the UK, focusing on the effects of Turkish supplementary schools and home language environments. A mixed-methods approach was used to compare 4–8-year-old children attending Turkish supplementary schools with those who do not, using vocabulary and narrative tasks and questionnaires. Participants included children from Turkish supplementary schools (attendees, n = 19) and Turkish communities (non-attendees, n = 22) in Doncaster, London, Manchester, Nottingham and York, along with their parents (n = 39). Data were collected in two phases. Initial results indicated that attendees underperformed in Turkish vocabulary production and comprehension, as well as narrative skills, compared to non-attendees, although their English proficiency was comparable. Cross-sectional analysis showed significant lexical growth for both groups from time 1 (t1) to time 2 (t2). Despite these gains, attendees continued to lag in Turkish language knowledge. Attendance at supplementary schools did not significantly affect English vocabulary knowledge. The underperformance of attendees in Turkish language tasks is attributed to greater Turkish exposure and use at home among non-attendees. These findings highlight the critical role of home language practice in bilingual development, suggesting that future studies should consider home language use in their assessments.