The present invention relates to single domain antibodies comprising at least one modification relative to the 4D5 antibody scaffold or human germline VH3 domain, the modifications selected from the group consisting of H35D, A78V, S93V, S93G and W103R, with the position numbering being according to the Kabat numbering scheme. Disulfide-free variants further comprise at least one additional modification selected from the group consisting of C22S, A24I, A24L and C92T, and with the proviso that at least one of C22S and C92T is present. Further encompassed are the multi-modular antibody molecules and antibody conjugates comprising single domain antibodies, as well as methods for producing them. The invention in particular provides a library of the single domain antibodies or multi-modular antibody molecules and a method for selecting an antibody that binds an antigen.