  Detached double white dwarf (DWD) binaries are one of the main science case
for the Laser Interferometer Space Antenna (LISA). As the most numerous LISA
sources, they will provide important contributions towards understanding binary
evolution, Supernovae Type Ia (SNIa) formation channels and the structure of
the Milky Way. So far only detection prospects for the Milky Way have been
computed. In this letter we show that LISA has the potential to detect DWDs in
neighboring galaxies up to the border of the Local Group. In particular, we
compute quantitative estimates for the number of detections in M31. We expect
between a dozen to several tens of DWDs above the nominal detection threshold,
for a mission duration between 4 and 10$\,$yr. We show that extra-galactic DWDs
detectable by LISA are those with the shortest orbital periods and with the
highest chirp masses, that are candidates SNIa progenitor, virtually
undetectable at those distances in optical. This implies that LISA will be the
only instrument able to provide SNIa merger rates across the Local Group.
