Complex building structures fundamentally limit indoor wireless performance. While the wireless industry consistently pursues optimal channel capacity, architectural constraints remain significant factors, largely neglected by conventional workflows. To bridge the gap, this paper expands the inter-disciplinary building wireless performance (BWP) framework by introducing a layout-specific evaluation of area spectral efficiency (ASE). We propose ASE gain as a novel metric to quantify the impact of building layouts on indoor wireless channel capacity. Furthermore, we derive and validate an analytical model for the ASE gain against Monte Carlo simulations. This approach significantly reduces the computation time from hours to minutes with a relative error of less than 1%. Numerical results demonstrate that the ASE gain is significantly influenced by building layouts and wireless parameters. For example, in multi-rectangular layouts, reducing the reception threshold density of 10 dBWm−2decreases the variation range by 0.2059 at 1 GHz, whereas it increases the range by 0.1990 at 28 GHz. For non-rectangular polygonal layouts, the variation range is one order of magnitude larger than in simple rectangular rooms. This work highlights the importance of integrating the ASE gain into future building design. It complements existing metrics such as power gain and interference gain to ensure that buildings provide sufficient margin for the future deployment of high-density networks.