  Spiral structure can occupy a significant part of the galaxy, but properly
accounting for it in photometric decomposition is rarely done. This may lead to
significant errors in the parameters determined. To estimate how exactly
neglecting the presence of spiral arms affects the estimation of galaxy
decomposition parameters, we perform fitting of 29 galaxies considering spiral
arms as a separate component. In this study, we utilize 3.6$\mu$m-band images
from the S$^4$G survey and use a new 2D photometric model where each spiral arm
is modeled independently. In our model, the light distribution both along and
across the arm can be varied significantly, as well as its overall shape. We
analyze the differences between models with and without spiral arms, and show
that neglecting spiral arms in decomposition causes errors in estimating the
parameters of the disk, the bulge, and the bar. We retrieve different
parameters of the spiral arms themselves, including their pitch angles, widths,
and spiral-to-total luminosity ratio, and examine various relations between
them and other galaxy parameters. In particular, we find that the
spiral-to-total ratio is higher for galaxies with more luminous discs and with
higher bulge-to-total ratios. We report that the pitch angle of spiral arms
decreases with increasing bulge or bar fraction. We measure the width of the
spiral arms to be 53\% of the disc scale length, on average. We examine the
contribution of the spiral arms to the azimuthally-averaged brightness profile
and find that spiral arms produce a ``bump'' on this profile with a typical
height of 0.3--0.7 mag.
