  Charge density waves are ubiquitous phenomena in metallic transition metal
dichalcogenides. In NbSe$_2$, a triangular $3\times3$ structural modulation is
coupled to a charge modulation. Recent experiments reported evidence for a
triangular-stripe transition at the surface, due to strain or accidental doping
and associated to a $4\times4$ modulation. We employ \textit{ab-initio}
calculations to investigate the strain-induced structural instabilities in a
pristine single layer and analyse the energy hierarchy of the structural and
charge modulations. Our results support the observation of phase separation
between triangular and stripe phases in 1H-NbSe$_2$, relating the stripe phase
to compressive isotropic strain, favouring the $4\times4$ modulation. The
observed wavelength of the charge modulation is also reproduced with good
accuracy.
