Research shows that children's block construction skills are positively associated with their concurrent and later mathematics performance. Furthermore, there is evidence that block construction training is particularly beneficial for improving early mathematics skills in children from low-Socio Economic Status (SES) groups who are known to have lower maths performance than their peers. The current study investigates (a) the association between block construction and mathematics in children just before the start of formal schooling (4 years-of-age in the UK) and (b) whether the association between block construction and mathematics differs between children from more compared to less affluent families. Participants in this study included 116 children (M = 3 years 11 months, SD = 3 months) who all completed numeracy, block construction, and receptive vocabulary tasks. Socio-economic status and demographic information (child age, gender, ethnicity) were also obtained from parents. Findings show a strong positive association between block construction and early numeracy skills. Block construction skills explained approximately 5% of the variation in numeracy, even after controlling for age in months, household income, and child receptive vocabulary. When separated by SES group, for children from less affluent families, block construction explained a significant amount of variability (14.5%) in numeracy performance after covariates. For children from more affluent families, block construction did not explain a significant amount of variation in numeracy. These findings suggest that, interventions involving block construction skills may help to reduce SES-based attainment gaps in UK children's mathematics achievement.