Washington’s Union Station is one of the city’s many registered landmarks protected because of their age and architectural characteristics. (cineman69/Getty Images)

Be grateful for historic preservation policies, laws and advocacy. They enable us to identify and protect America’s significant cultural heritage, ensuring that future generations will continue seeing, using and appreciating meaningful legacies of the past.

Preservation efforts focus on architecture; parts of towns, cities and neighborhoods; natural and designed landscapes; civic spaces and works of public infrastructure; and places where important historic events, such as Civil War battles, occurred.

Thanks to historic preservation legislation, Georgetown, Capitol Hill, Old Town Alexandria, and the Mall’s memorials and museums are deemed historic. Cleveland Park, Chevy Chase and Annapolis are designated historic districts.

Countless buildings in Washington — the U.S. Capitol, Union Station, hotels, churches, institutional buildings, schools, libraries — are registered landmarks, not only because of their age but also because of their notable architectural characteristics and quality.

Since World War II, awareness of historic preservation and regulations promoting it have evolved to be among the most potent forces affecting urban planning, real estate development, building design and landscape architecture.

Today developers are increasingly required to diligently address questions about what already sits on, around or beneath a project site.

Are there existing structures or artifacts deserving to be wholly or partially saved as landmarks for compelling historic reasons?

Can a historic resource be modified and repurposed without losing its historic character? Is a structure’s partial or complete demolition necessary and justifiable because physical and environmental deterioration make it unstable, unsafe and beyond technically or economically feasible remediation and reuse?

Despite evidentiary agreement as to the history, condition and durability of a structure, credible yet contradictory interpretations of such evidence are common. Indeed, deciding whether and to what extent an aging resource should remain in its original form entails subjective value judgment. It is a matter of informed opinion and not just factual analysis.

Landmark designation doesn’t irrevocably prevent modification or demolition. But it does mean that altering or razing a registered historic landmark cannot be undertaken as a matter of right. To make such changes to a historic property, an owner or developer must seek legal authorization based on a lengthy, demanding entitlement process. Thus, decisions about a proposed transformative project affecting a historic landmark can be controversial and take years to resolve at great expense.

The fate of the Third Church of Christ, Scientist sanctuary formerly at 16th and I streets NW is a case in point. Designed and built in 1971 by one of America’s most esteemed architecture firms, I.M. Pei & Partners, the octagonal concrete structure had been landmarked as an important example of 20th-century architectural Brutalism.

However, the church sought to demolish the building, which the congregation had come to dislike intensely for aesthetic, functional, operational and financial reasons. A drawn-out and costly legal battle ensued, pitting preservation interests against the church and its development partner. The dispute ended when the city ruled in favor of the church, and demolition occurred in 2014.

Under federal, state and local historic preservation statutes, rigorous standards exist for determining historic landmark eligibility of a property; for granting historic preservation designation and registration; and for authorizing physical alterations or demolition of all or part of a property that is historically registered.

An appointed state historic preservation officer and a staff of preservation professionals are responsible for implementing and enforcing historic preservation laws. In Washington, the historic preservation officer and staff provide support for the city’s Historic Preservation Review Board (HPRB). Based on testimony at public hearings and the preservation officer’s recommendations, the HPRB rules on landmark nominations and proposed changes to historic landmarks.

Despite the detailed preservation standards and legislatively mandated review process, the regulations can be used to delay, disrupt or stop projects affecting older structures for reasons unrelated to safeguarding historical legacies.

When sincere, devoted preservationists seek to save venerable structures or historic elements of structures, they do so to prevent a tragic cultural and societal loss.

But others espousing preservation may oppose a proposed project not because of historic concerns but because they fear it will negatively affect their property or neighborhood.

Consequently, historic preservation authorities have an especially fraught responsibility. They must thoroughly probe the evidence and arguments and then render an often debatable, unprovable value judgment. That is why historic preservation judges need to be knowledgeable, critically thinking and thick-skinned.

Roger K. Lewis is a retired practicing architect, a professor emeritus of architecture at the University of Maryland and a regular guest commentator on “The Kojo Nnamdi Show” on WAMU (88.5 FM).