In most jurisdictions in the US, permits are required for anything involving plumbing and electric.

"Brace the roof and just replace the two walls and floor" may be "repairs" to you, but these are structural additions to your home, and this work requires permits.

At a minimum, permits ensure that your plumbing will be vented and tied into the system correctly, electric lines and connections are tied in properly and safely and meet the building codes, and there will be inspections to document that.

Your roof needs bracing? I'd want to be sure it wasn't going to collapse when the washing machine gets off balance and vibrates itself across the room. That has got to be done right, and confirm that the loads are supported properly.

If the contractor wants to skimp on permits, where else is he going to skimp? Any contractor that cuts corners here is putting not just your home, but you and your family's lives at risk.