As soon as you request a different grade of material than is offered by the builder and included in the base price of the home you are requesting an “upgrade”.
The builder expects you to do this and makes certain “options” available to the buyer by pre-pricing the expected upgrades.
For example: the builder has already priced groups of tile in amounts necessary for each of the areas in the specific plan you are purchasing, they know what it will cost to convert that open den to another bedroom etc.
These upgrades and options include everything from door knobs, lighting, windows, flooring and cabinets to raising a ceiling, rounding corners, adding a bathroom or converting a garage. The average buyer will spend from 15% up to 30% or more of the base price in upgrades. More if they add a casita or pool. Although the builders do not as a rule build pools, they work closely with selected pool contractors and the cost of a pool can usually be added into your mortgage.
Any type of renovation will need to be thoroughly looked through to make sure it fits in with the current standing home as well as the area. If it does not then it will either be harder to construct with the specifications and materials at hand or it will not be able to be built. If a renovation/upgrade is approved, such as an ADU (accessory dwelling unit) from companies like United Dwelling, then step-by-step plans will need to be followed and a price point set out before any construction takes place. Talk through the options laid out and see which is the most beneficial for the current situation, after that step has been approved along with the financials and a time frame, it will then be ready for the project/s to start.