Abstract A first-principles approach for active chiral hard disks is presented, that 
explicitly accounts for steric interactions on the two-body level. We derive an 
effective one-body equation for the joint probability distribution of positions 
and angles of the particles. By projecting onto the angular modes, we write a 
hierarchy for the lowest hydrodynamic modes, i.e. particle density, polarisation, 
and nematic tensor. Introducing dimensionless variables in the equations, we
highlight the assumptions, which - though inherent - are often included implicit in 
typical closure schemes of the hierarchy. By considering 
different regimes of the Péclet number, the well-known models in active 
matter can be obtained through our consideration. Explicitly, we derive 
an effective diffusive description and by going to higher 
orders in the closure scheme, we show that this first-principles approach 
results in the recently introduced Active Model B +, a natural extension of the 
Model B for active processes. Remarkably, here we find that chirality can 
change the sign of the phenomenological activity parameters.