The Home Learning Environment (HLE) focuses on everyday learning habits in families to support the development of children’s early numeracy and literacy competencies. In recent years, a growing number of studies have assessed the HLE by using different conceptual approaches and various assessment methods, often focusing on either the home literacy environment or the home numeracy environment. However, it is still unclear whether the two dimensions of the HLE are separable constructs and which assessment method is best suited for assessing the HLE, making it difficult to interpret and compare studies on the HLE’s relation to children’s competency development. In the current study, we used multitrait-multimethod (MTMM) analyses to compare three common methods for assessing the HLE and their relations to various linguistic and mathematical outcomes of kindergarten children: parental surveys, observations of a caregiver-child interaction, and title recognition tests (TRT-VS, MTRT). Our sample consisted of N = 190 children with an average age of 64 months (SD = 4.4). All HLE measurements were significantly intercorrelated. The MTMM matrix showed a substantial effect of common methods and indicated a one-dimensional HLE construct. The children’s book title recognition test (TRT-VS) was the best predictor of children’s linguistic and numeracy competencies with a factor loading of λ = .95. Even when controlling for characteristics of the child and the family (e.g., children's intelligence, parental socioeconomic status), the HLE was significantly related to both the mathematical competencies (ꞵ = 0.28, p < .001) and the linguistic competencies of the child (ꞵ = 0.65, p < .001) in a structural equation model. We discuss the significance of a one-dimensional HLE construct and its different assessment methods in the light of implications for future research and interventional approaches.