A method for assembling blade parts of a wind turbine blade includes the steps of: obtaining information about a connection area of a blade part, at least one of selecting, customizing and manufacturing an adaptor piece depending on the obtained information about the connection area of the blade part, wherein the adaptor piece serves to connect the blade part with at least another blade part, and connecting the blade part to the adaptor piece. Independently manufactured blade parts of a wind turbine blade are assembled in a manner such that the assembled blade comes as close as possible to a single-casted blade.