  We present $J$-band near-infrared (NIR) imaging of the host galaxies of nine
narrow-line Seyfert 1 galaxies (NLS1). Based on high-frequency radio
observations at 37~GHz, seven of them could host powerful, most likely
relativistic jets. Host galaxy morphology studies of NLS1 galaxies are scarce,
but exceedingly important for understanding the seemingly heterogeneous nature
of the NLS1 population as well as their evolution and place in the active
galactic nuclei (AGN) scheme. Increasing the sample size is essential for
achieving statistically significant results. We determine the morphological
types of the host galaxies by performing photometric decomposition of NIR
images using a 2D image decomposition algorithm GALFIT. We were able to
sufficiently model five of the nine host galaxies. Based on the fitting
parameters, mainly the S\'{e}rsic index, all of them are disk-like galaxies.
Sources with clearly distinguishable bulge components all have pseudo-bulges,
and four out of five sources show a component resembling a bar. A surprisingly
large fraction, three out of five, show signs of interaction or disturbed
morphology. Our results suggest that spiral galaxies with pseudo-bulges are
able to launch and maintain powerful jets. They also imply that interaction -
mainly minor mergers - may have a role in initially triggering higher levels of
nuclear activity in NLS1 galaxies. Furthermore, our results support the
heterogeneous nature of the NLS1 class and indicate that this diversity is
caused by different evolutionary stages, possibly due to mergers.
