The 2nd and 3rd floor needs rehab work including, - Full Painting Flooring (Vinyl/Linoleum)- Replace Kitchen cabinets- Renovate the Bathrooms (replace tiles)- Drop a wall and make a corridor- Electrical throughout My question is, do I need to apply for a permit for all of this work?

You can maybe find a duplex that needs some cosmetic upgrades (think cabinets, flooring, old appliances) and add value to it, without having a huge rehab project.

I rehab with all cheap flooring, cabinets etc and rent to Section 8 tenants only in this area.

We just fully renovated the property, new kitchen cabinets, countertops and backsplash.

A body smell can permeate walls, cabinets, wood, cement. 

Same with the cabinets.

If the cabinets are gross and going to keep people from renting unit, you need to replace them, even if they won't get a bump in rent, because no one is renting an apartment without kitchen cabinets.

Renovations consisted of lvp flooring , inside outside paint, new appliances, kitchen and bathroom cabinets, new stone countertops , updated electrical and plumbing fixtures, new hvac, and new water heater.

You have basic finishes like laminate counters, 30" cabinets, a coil stove a TV sitting on a bar stool and your pool doesn't have a hot tub, that alone is losing you bookings. 

But changing cabinet pulls, fresh paint on trim.