Nowadays, sewage sludge (SS) disposal is an urgent problem that needs to be solved. Thermal utilization is the preferred method to be employed, but it faces some significant operating problems. Firstly, it is necessary to apply a very costly predrying scheme to the sewage sludge, a municipal wastewater plant by-product characterized by its high moisture content. Secondly, its high heating value (HHV) is very low which results in an inefficient and high emission combustion process. That is why the application of hydrothermal co-carbonization (co-HTC) of sewage sludge and fuel characteristic additives can solve these problems. This study is focused on the fuel properties of hydrochars produced in such a reaction. Hydrothermal co-carbonization of sewage sludge with charcoal (10% db), oak sawdust (10% db) and fir sawdust (10% and 20% db) was conducted. Prior to and after the HTC process, the physical and chemical properties were analysed to examine the changes in the composition of the studied materials. The ultimate and proximate analyses, mass and energy yields, HHVs and BET of hydrochars were determined. The XRF analysis of ash received from raw materials was conducted. In addition, the combustion and comprehensive combustibility indexes were calculated based on TGA. Concluding, the addition of biomass additives to sewage sludge in the co-HTC process were proven to increase the HHV of hydrochars. The best results were found for a 20% addition of fir with 16.038 MJ/kg as the outcome. Moreover, all additives are believed to provide a more stable combustion process demonstrated by higher values of carbon content (from 34.9% to 37.9%) and lower values of volatile matter (from 56.4% to 40.7%). Furthermore, the ash content during the hydrothermal treatment of sewage sludge usually increases but the presence of additives moderated this unwanted effect, namely the ash yields for hydrochars derived from sewage sludge and additives were 40.7-44.5%, whereas, for hydrochar of sewage sludge it was 52.0% and for untreated SS it was 36.5%.