I've an Ateca and a Renault Megane. Last year I had both Renault front coil springs relaced for £220 total. I later had to have the rear replaced, therefore it looks as though once one is corroded and breaks, then the others will probably follow.

 

It is normal both sides to be replaced. Coil springs will sag a bit during their lifetime.

 

Yep, normal to replace road springs in pairs - in truth they are colour coded, however after 7 years of road grime one would be very lucky to see the dabs of paint so they are always ordered as a pair.

 

Conventional wisdom states that most suspension parts should be replaced in pairs, unless the car is very new.
Handling and ride quality can be effected if only replacing one side.