  Aims. Hot accretion flows are thought to be able to power the relativistic
jets observed in Active Galactic Nuclei. They can present themselves as SANE
(Standard And Normal Evolution) disks or MAD (Magnetically Arrested Disks), two
states implying profound differences in the physical properties of the disks
themselves and of the outflows they produce. Methods. In this paper we use a
multi-frequency and multi-epoch data set to study the giant radio galaxy NGC
315, with the goal to explore the properties of its accretion disk and
sub-parsec jet. We analyze the source maps with a pixel-based analysis and we
use theoretical models to link the observational properties of the jet to the
physical state of the accretion disk. Results. We propose that the bulk flow in
NGC 315 accelerates on sub-pc scales, concurrently with the parabolic
expansion. We show that this fast acceleration can be theoretically reconciled
with a magnetically driven acceleration. Along the acceleration and collimation
zone, we observe an unexpected spectral behavior, with very steep spectral
index values $\alpha \sim -1.5$ ($S_\nu \propto \nu^\alpha$) between 22 GHz and
43 GHz. Based on the properties of this region, we predict the black hole of
NGC 315 to be fast rotating and the magnetic flux threading the accretion disk
to be in excellent agreement with that expected in the case of a MAD. Using a
new formalism based on the core-shift effect, we model the magnetic field
downstream a quasi-parabolic accelerating jet and we reconstruct it up to the
event horizon radius. In the MAD scenario, we compare it with the expected
magnetic saturation strengths in the disk, finding a good agreement.
