Residential domestic hot water (DHW) energy consumption represented 16% of the EU household heating demand in 2013. Due to the improvement of the building envelop, it is expected to increase significantly, with values between 20% to 32% in single family buildings, and between 35% to almost 50% in multifamily buildings. Currently, this energy is lost to the environment after its use, but it can be recovered by waste water heat recovery (WWHR) systems inside buildings (in-building solution). However, the potential of such solutions has not been assessed in detail for different types of buildings or at urban scale. Also, the characterisation of waste water streams has barely been addressed. A method quantifying the energy saving potential at urban scale of in-building WWHR systems in residential buildings is therefore proposed. The characterisation of residential waste water streams as to mass flow and temperature level is also addressed. The method is applied to a real case-study, where the impacts of shower and grey water heat exchangers are assessed. Grey water heat recovery for hot water preheating yields up to 18% and 27% fuel savings for passive single family houses and multifamily buildings, respectively. With the detailed characterisation of the waste water streams, the quantification of the energy savings through heat recovery is improved. The energy savings achieved by in-building WWHR systems can be more precisely compared with other optimisation measures. The outcomes of urban energy assessments concerning waste water heat recovery are also improved, as the results at building level are aggregated to the considered geographical scope. The proposed method therefore complements current urban energy assessments with a detailed analysis of in-building waste water heat recovery systems.