There's many ways that the size of your new home is limited, from the the floor area to roof height. Your local Zoning Department will use many different methods to govern your home size - and it varies per City!
Let's take a look at the main ones.
Floor Area and Coverage
Floor Area is essentially a measurement of the floor space of your home. It's measured in Square Feet (SF) and there are many methods in how to calculate. It may be comprising only the habitable area inside your home, or may include the exterior walls encompassing it.
Floor Area Ratio
Floor Area Ratio (or FAR) is defined by a ratio or formula that is dependent on your Lot/Yard Size. It is the total habitable area, each story, typically measured from the exterior face of framing. It can also be described by Net Area, Usable Floor Area, Habitable Area, and Residential Floor Area.
For example, a 10,000 SF Lot with a 4:1 FAR would mean the Floor Area would be limited to 2,500 SF.
Building or Lot Coverage
Typically called Gross Area, Building Coverage, or Lot Coverage, this limit is based on the size of your Lot (seeing a pattern?). It's the total size of the House that is touching the earth and can include covered porches, raised decks, pergolas, and garages. I tend to think of it as the "size-by-satellite".
This limit is typically as a percentage, so a 10,000 SF Lot with a 40% Coverage would mean 6,000 SF of the yard or lot remains unoccupied.
Usually basements are excluded from Floor Area calculations, depending on how much of the basement is fully (or mostly) underground. This is due to the floor area being "invisible", and Walk-Out Basements will generally count as Above Grade Area or Floor Area.
Principal vs. Accessory
There are two types of structures on your Lot and there are different limits for each. The main house/structure is the Principal, while all of the secondary buildings like Garages, Sheds, Barns, Pergolas are Accessory. In most cases you cannot have an Accessory Building without the Prinicipal and the Accessory cannot exceed the Prinicipal.
Setbacks and Easements
Each Lot has an amount of buildable area which is defined mainly by Setbacks and sometimes Easements. It's present on every Lot and each side and varies for the structure type and the Lot's zoning. It's there to maintain and preserve yard space between Homes, so your neighbor doesn't build right on your property line. It can also have an aesthetic goal, such as keeping neighborhoods aesthetically pleasing with aligned front porches.
Easements are secondary setbacks which prevent any structures for occurring on it. It's there to preserve space or access and includes Utility Maintenance, Drainage, or driveway and access. They can be established by your Utility Company, Building Department, or your Neighbor. All Lots will have Setbacks, but not all Lots will have Easements.
Bulk Plane and Height Limits
We've discussed Floor Area and Setbacks so let's move on to the third limit which is Height. Like all the Limits discussed, this is based on the Zoning District and if it's a Principal or Accessory Structure.
As the name suggests, it's the maximum height for your home. While it can include the maximum stories or levels, it is generally the measurement from grade (ground) to the highest point of the structure.
This is a combination of the Side Setback and Maximum Height and governs the "bulk" of the house. It's goal is to limit the structure height along the side, to allow for daylight and open space, thus preventing your modest 1 story home from neighboring a solid 30' wall.
Usually it's an imaginary 12' wall from the Side Setbacks or Property Lines, then a 45 degree angle to the maximum height.
Unique to City of Boulder, this is a height limit directly related to allowable sunlight. It's intended to prevent neighbors casting extensive shadows onto your property. One can imagine the frustration of having your backyard solar or garden never seeing the light of day. While it's a complicated measurement, everyone deserves sunlight.
Every location is different in how they limit the size of homes. Some are code-driven while others require a Planning Review to determine the maximum size. There could also be additional limits from a Home Owners Association (HOA), Covenant, or Planned Unit Developments (PUDs).
Fortunately, we're here to help! We're happy to check what limits your home or future project!