Recent developments in building energy models for urban energy simulation are primarily based on bottom-up modelling (N models used for N buildings). This work aims to develop a single assembled model for multiple buildings for convenient use in detailed urban analysis. The proposed model exhibits state-space model formalism, and a state-size reduction technique is applied to maintain model accuracy, even for a low-order representation. To accelerate the calculation time and ensure numerical stability, a direct solver is proposed to eliminate the iterative calculations required in Dymola for annual load calculations. The results of the proposed reduced model are in good agreement with the reference model. For a test case of ten buildings, a 2nd order reduced model (i.e., 2 differential equations) with the proposed direct solver can predict accurately the dynamic energy behaviour, resulting in an error of about 0.43% for the annual loads.