Homeowners might be annoyed if local zoning ordinances require testing and possible upgrades to septic systems if more bedrooms are being added to a home. There are, however, very good reasons for those requirements. Each person added to a household adds to the water usage of that home. Septic systems are designed to handle a certain amount of water, and exceeding that amount of water can cause serious problems.
A septic tank works by allowing solids to settle to the bottom, where they are naturally broken up by bacteria. As water rises in the tank, it flows out into a drain field. Overloading the tank causes more solid material to settle in the bottom than will break down readily, and can stir up those solids. An overloaded tank allows those stirred-up solids to flow out into the drain field, contaminating it.
The drain field works by allowing the water to gradually seep through filtering layers into the ground, where it enters the area's ground water. Too much water flowing into the drain field will force contaminated water into the local ground water, and will cause the drain field to become boggy with dirty water at the surface. This creates a disgusting, unsanitary, stinking mess an extra return.
If you have any concerns about whether your septic system is capable of handling the amount of water your home uses, consult qualified experts, like the folks at All About the Pipes.