Trees are one of the few carbon sinks in urban areas. Different methods are available to assess biomass of urban trees, one of it being biomass functions. One advantage of biomass functions is their easy and low-cost application because required data like diameter and height are directly available from tree inventories. Our data show that it is not suitable to use forest tree biomass functions assize and biomass allocation differ between both ecospheres. Hence, it isimportant to apply specific urban tree biomass function if biomass or carbonstorage is of interest. We started to develop new urban tree aboveground biomass functions using 144 measured deciduous trees of fourteen tree species inKarlsruhe, Germany. Conifers are also of interest but not covered in this data, so we explored several possibilities to build more general models incorporatingdeciduous and conifer tree species from urban and forest origin. Among others, we tested adjusted forest biomass models and a cross-classified mixed model using data from urban and forest origin holding more than 2200 conifer and deciduous trees. This last model shows best predictive performance for deciduous urban tree species, assured by ten-fold cross-validation on group- andpopulation level. We also compared performance to conifer forest biomass functions, showing slightly improved BIAS values. As a feature, the model isalso able to make predictions also for non-observed conifers in urban space, under the assumption of comparable urban-forest differences between deciduousand conifer species. A sample application shows results for a small subset ofdata of a urban tree inventory, collected in a residential area in the city ofMunich, Germany.