The solar fence, explained
Boulder regulations protecting the use of solar energy sit at section 9-9-17 of the Boulder Revised Code 1981, and they limit the extent to which new construction and additions may shade adjacent properties.
The protection is defined by a hypothetical solar fence on the property lines, either 12 feet or 25 feet depending on the applicable area. New building elements are generally not permitted to shade adjacent properties to a greater extent than that fence would.
The test is specific rather than seasonal in a vague sense. It protects a four-hour period, from two hours before to two hours after local solar noon, on December 21, which is the worst-case day for a solar array in the northern hemisphere.
The practical rule is simple: if all shadows fall within the property lines the building complies, and any element casting a shadow past the property line is potentially a violation. That is a genuine constraint on your neighbours and, equally, a constraint on you if you are planning an addition.
When the automatic protection is not enough
The solar fence protection applies automatically. Where an owner has installed or plans to install a solar energy system and needs more protection than that, a solar access permit is available, and it is valid for the reasonable life expectancy of the particular solar energy system.
That is a meaningfully stronger position than most homeowners anywhere have. It converts a hope that the surroundings will not change into something recorded and enforceable for the life of the array.
A variance to the solar access regulations, called a Solar Exception, can be approved only where an application meeting all requirements is submitted, the affected property owner gives permission, and the applicable exception criteria are demonstrated.
Ask your installer whether your design would benefit from a solar access permit, particularly if a neighbouring lot is undeveloped, has an older single-storey house, or sits directly south of your array. The time to ask is during design, not after a shading problem exists.
Why shading deserves this much attention
Shading is not proportional in the way people assume. Depending on how an array is wired and whether module-level electronics are used, shade falling on a small part of an array can cost far more than that fraction of its output.
That is why the December 21 test in the Boulder ordinance is well chosen. It targets the day when the sun is lowest and shadows are longest, which is when a tree or a neighbouring roofline does the most damage to a winter production figure.
Ask your installer for the shading analysis behind the production estimate, and ask specifically what it assumed about vegetation growth. Trees are not static, and a model built on today canopy will overstate production in ten years.
Ask also whether module-level power electronics are included in the design. In a city with mature tree cover and a genuine winter shading profile, the case for them is stronger than it would be on an unobstructed suburban roof.
The parts that survive without qualification
The 30 percent federal Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, so a cash or loan purchase in Boulder receives no federal tax credit. A quote that still applies one is overstating your return by roughly a third.
Section 48E, the commercial credit, survives at 30 percent and is available to third-party owners under leases and power purchase agreements. Ask what the provider claims and what portion reaches you in the rate offered, and confirm the treatment with a tax advisor.
What remains is full retail net metering with the Solar Bank election, Solar*Rewards for Xcel customers in exchange for your Renewable Energy Credits, the property tax exemption for residential systems of no more than 100 kW AC under Section 39-3-102 C.R.S., and the residential energy sales and use tax exemption.
Ask any installer to rebuild the projection from those alone, and to show the shading analysis that produced the annual output figure. In Boulder the production assumption deserves as much scrutiny as the incentive assumptions.