The City of Berkeley is currently updating their accessory dwelling codes and height is an issue that is being reviewed. Many cities struggle with this. This is a summary of what you get for 14′ vs. 18′
To calculate the maximum height you measure the height of a building from the soil to the peak of the roof.
An 18′ tall design the foundation is 1 foot, the first floor is 8 feet, a second floor is 7 feet, the floor between the two is 1 foot and the roof is 1 foot. That adds up to 18′ total. You get a decent second floor with a 7 foot height at the peak in this design.
In the 17′ tall design below you can see that the peak of the upper floor is 5’11”. This design is for a guy who is 6’8, so we have a major design flaw forced on us by the height limitation.
In the seconds design below, we were constrained to a 15′ height.
The owner here insisted on an above ground crawl space. Owners that don’t listen to our advice is just one of the countless challenges architects and contractors have, that’s another story though:)
In this design, the foundation is 1.5 feet. The ground floor height is 7 feet 6 inches. The floor thickness between the two floors is almost 1 foot, the upper floor is 5 feet 1 inch and the roof is one foot thicks. 1.5+7.5+1+5+1 = 15 feet total height.
This owner was not happy to have a 5 foot 1 inch tall upstairs. No one was happy with a 7 foot 6 inch main floor. Excavating and creating a dug out lower floor can solve this but then you have four retaining walls and a concrete floor and moisture issues that get expensive and frankly, it’s not that nice to walk down three of four steps to your main floor.
Adding a second floor can cut the per square foot construction cost in half – from $500 per square foot to $250 per square foot. That per square foot cost is how construction is measured and it is one of the best ways to create valuable usable space affordably.