Boulder’s Solar Shadow Ordinance

Many decades ago, in an effort to ensure that the City of Boulder’s citizen’s houses always have the access to the sun, the City passed a Solar Shadow Access ordinance. If you have done any building work in town you have certainly run into this rascal as it applies to all residential districts and can be particularly tricky.

NOT ALL PROPERTIES ARE THE SAME This solar protection rule projects the potential shadow from new construction as calculated between 10am and 2pm as it would be on December 21st. This means that the shadow is a wedge casting north and 30 degrees east and west of north. So, if you live on the south side of an east-west street, your shadow casts mostly into the street. If you live on the north side of an east-west street you are a bit disadvantaged as your shadow most likely begins to violate the regulation by casting into your neighbor’s property. On north-south streets, this regulation pushes new work to the south side of the property to avoid solar trespass at the north edge.

available building area, north-south vs. east-west street

THE SIMPLE ANALYSIS In its simplest form, different areas of the city are divided into Solar Access Areas, one having a 12′ high solar fence, one a 25′ high solar fence. A solar fence is basically a theoretical plane of described height on the property lines. Based on a formula of pitch, you can build new construction as long as the pitch of the shadow of the new work does not cross your property line. The formulas are based on the assumption of the height of your Solar Access Area’s fence. That’s the simple part.

actual and solar fence site plan analysis

THE PAIN-IN-THE-BUTT ANALYSIS If your proposed work’s shadow as defined by the formulas described above, does NOT fall within your property lines or your lot is not “level”, you have to execute a solar shadow analysis based on Actual Shadow Lengths. A “level” lot is a point of contention as you can imagine and the process for creating this analysis is essentially to plot the shadow lengths twice, once for a level lot, then adjust the length of the shadow based on the change of grade between the end of the shadow and the projecting element. What is particularly tricky about this is the impact of the location of your neighbor’s house relative to their setbacks. Your new construction can only cast as far into their property as they are NOT allowed to build, in other words, it can fall into their setback but no further. Many zoning districts in Boulder have a rule that the combined sideyard setbacks of a property need to be 15′, but either side can be as small as 5′ as long as the total is 15′. If your neighbor’s house, on the north side your property conjoins, is 5′ away from the property line, then the Actual Shadow Length restriction can be a project killer. If your property is also on a north-south street and fairly narrow, you’re done in.

blue: actual shadow plane; green: solar fence plane

So, how do you figure out what you can build? We create 3D computer models, and reverse engineering the shadow, to determine the available building envelope. This can be very tricky or very simple based on the orientation of the lot and house, the neighbor’s property, etc. Can you or anyone else simply come out to your property, take a look-around and give you an answer – NO. Believe me, this is not a pitch for trying to get more work as architects. I am pretty sure that no architect likes doing this stuff, it certainly isn’t why I spent 7 years getting Bachelors and Masters degrees.

Now that you have done all that work, you simply have to overlay the Bulk Plane restrictions, wall articulation requirements, the maximum building coverage restrictions, the Floor Area Ratio restrictions, any easements, flood zone restrictions and of course all setbacks and building height restrictions and you have a clear picture of what is your potential building. Easy. Do all these restrictions help protect our neighborhoods – yes. Are they restrictive and simple to figure out – no.

For the past number of years now the costs for installing photovoltaic panels have dropped significantly and the rebates have made this work an almost standard part of most of our projects. It appears that the City’s forethought all those many years ago was indeed brilliant prognostication. As you do not currently take into account the shadows of surrounding trees (although your solar panel provider certainly does), let’s hope that the pending Tree Protection ordinance does not make this all more complicated.


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