Adding space in Washington Park and south-central Denver

Washington Park, and the neighborhoods that ring it (Wash Park West, Platt Park, Bonnie Brae and the blocks toward University), is where Denver's appetite for more space meets its smallest older houses. The original stock is mostly early 20th-century bungalows, Denver Squares and cottages on narrow lots. Many have been scraped and replaced with new two-story homes. The ones that remain are usually being enlarged: up with a pop-top, back with a rear addition, or down with a finished basement.

All of it happens inside the City and County of Denver, so permits come from Community Planning and Development. What makes these projects hard isn't the permit office. It's the zoning envelope.

The envelope: base plane and bulk plane

Denver's zoning code limits how tall and how bulky a house can be through two connected ideas, explained in the city's Zoning Tip Sheet 2:

• Base plane. Height is measured from an imaginary plane tied to the original grade of the property, not the finished grade. The city says that is meant to stop owners from gaining height by regrading. Front and rear base planes are set separately.
• Bulk plane. From the side property lines, an imaginary plane rises vertically to a set height and then slopes inward. The building has to fit underneath it. Each zone district and building form sets its own numbers.

On a narrow Washington Park lot, the bulk plane is often what limits a pop-top, not the overall height limit. It's the reason so many second stories here have sloped side walls, dormers or a stepped-in upper floor. Before you pay for design, get your zone district from the city's zoning map and have the architect run a quick bulk plane study.

Denver's Permitting Checklist for New Homes and Additions spells out what the plans must show: maximum height measured from the front and rear base planes, the bulk plane for each structure, and upper-story setbacks where they apply. The same checklist says all new homes, and some addition projects, require a survey and a geotechnical soils report. Ask early whether yours will. A soils report takes time to schedule.

Pop-top, rear addition or ADU?

Pop-top. It keeps the yard, but it puts the most load on the existing structure. Expect a structural engineer to check the footings and walls and specify reinforcement, a new stair that eats floor area on the main level, and, typically, moving out for part of construction.

Rear addition. It's structurally simpler and lets you stay home longer, but it uses yard space and still has to meet rear setbacks and the rear base plane rules.

ADU. Since Denver's Citywide ADUs amendment took effect on December 16, 2024, the city says ADUs are allowed on about 70% of Denver's land. A detached unit over a rebuilt garage on the alley can add a home office, guest space or rental without touching the main house.

Our pop-tops and additions guide covers costs and the design process in detail.

Going down instead of up

Basements are the cheapest square footage in an older Wash Park house, but they come with limits. Many original basements are shallow, with low ceilings, a furnace and water heater in the middle and small windows. Finishing one may involve:

• An egress window with a proper well for any bedroom.
• Moving or replacing ductwork to gain ceiling height.
• A radon test first. The Colorado Department of Public Health and Environment says about half of Colorado homes test above the EPA's 4 picocuries per liter action level.
• Lowering the floor (underpinning), which is a major structural project that needs an engineer, not a basement finisher.

Old-house items to budget for

Before adding space, deal with what the existing house needs. Common items here include replacing galvanized water lines, scoping the clay or cast-iron sewer lateral (in Denver the lateral is the owner's responsibility up to the main), upgrading an undersized electrical service, and replacing an aging furnace or adding cooling. If an addition will need a new panel or heat pump anyway, plan those systems for the whole house at once.

Practical advice for this neighborhood

• Talk to neighbors before construction. Narrow side yards mean staging, scaffolding and access often require cooperation.
• Expect the city to look closely at drainage and grading on a lot that's being built up.
• Put the structural engineer's report, the zoning study and the soils report (if needed) in front of every contractor you ask to bid, so you're comparing the same scope.