This paper explores the possibility of characterising the degree of fenestration of a certain city by studying a limited building sample. For the case study of the Eixample district of Barcelona, window-to-wall ratio (WWR) guiding values are provided and the role of the date of construction and the façade orientation is assessed regarding the opening size. Results indicate that not only average WWR values but also dispersion indicators should be provided to adequately describe façade fenestration. The age of construction has found to be a helpful parameter to approximate representative WWR values of a tissue reducing the uncertainty.