Swedish students learning English as a foreign language (L2) are increasingly exposed to English outside the classroom, referred to as Extramural English (EE), through activities such as gaming and watching YouTube videos, yet most previous research is limited to what happens exclusively in the classroom. Situated within usage-based theories of L2 acquisition, this project examines the impact of five EE activities on students’ L2 writing development. To do so, we have compiled a corpus of 1,098 texts written by students in school years 9–12 in Sweden; unlike other learner corpora, the corpus also contains data on the students’ exposure to EE activities. We use a cross-sectional design and measures of linguistic complexity to operationalize and study writing development from three complementary perspectives: lexis (individual words), phraseology (word combinations), and grammar. The results from a pilot study show (i) that EE has a positive impact on lexical complexity, and (ii) that reading in English seems particularly helpful for promoting grammatical complexity in writing. Through addressing some hitherto under-researched areas such as the relation between EE activities and students’ L2 production and complexity development across levels, the project expands our understanding of the role of these informal, self-initiated language activities in L2 learning and has practical implications for the teaching of English in Swedish schools.