Historic homes (Roger K. Lewis/Roger K. Lewis)

So you want to acquire and live in a historic house, perhaps a “fixer-upper” in disrepair for sale at a favorable price?

This is an understandable, commendable aspiration in historic metropolitan Washington, where so much of the residential fabric encompasses diversely styled 18th-, 19th- and early 20th-century homes. Anyone committed to owning and restoring a historic home makes an important cultural contribution to preserving the city’s architectural legacy and venerable neighborhoods.

But taking on restoration and renovation of an aging structure, while a labor of love, also can feel like the labors of Hercules.

Before buying an older house, first hire a professional consultant with expertise in historic preservation and construction technology. The consultant will thoroughly survey and analyze physical conditions of the home’s exterior and interior components, from foundations to roof, from windows and doors to mechanical and electrical systems. You may be surprised by the inventory of problems and deficiencies.

Is there insect or animal infestation and damage? Is there asbestos insulation, or for that matter any insulation? Where is water getting in? It always gets in somewhere. How sound are the structural framing and bearing walls, and how much rotting, cracking and settlement have occurred? What kinds of paints were used? Any toxic substances to worry about? Are there prospective building code issues?

These are just a few of the questions a rigorous survey must answer. And each answer has significant financial implications, since money eventually must be invested to remedy or eliminate whatever deficiencies exist.

Research by the consultant also can ascertain the neighborhood’s and building’s historic “pedigree” describing its original design and authorship, including subsequent changes — for better or worse — to the original design. This research is indispensable for proper building restoration and is required for a building to be historically registered and landmarked.

Knowing the home’s history and physical conditions, you can undertake the challenging but stimulating task of deciding how specifically to stabilize, restore and, through appropriate redesign, modify the house for your use. This is when to consider contrasting preservation and renovation tactics about what to save and restore through rebuilding or refinishing; what to demolish; and what new construction to insert or add to create wonderful interior spaces. This is when you will need a good architect, a good contractor and, of course, a good budget and source of funds.

Tactical decision-making about preservation and restoration necessitates determining what specific physical elements of a building significantly contribute to its historic architectural form and character. Discarding or radically changing contributing elements clearly changes the architecture. For example, altering the position, size, dimensional proportions and style of windows on a street facade is tantamount to a major facelift. But replacing broken or missing roof shingles, modernizing a bathroom, adding closets, inserting a new structural beam or sanding wood floors is not.

Thus, deciding what to do with a historic house exterior worth preserving is relatively easy, typically involving remediation, refinishing and perhaps replication and replacement of existing historic elements, such as windows. Rotting, warped, loose windows and window frames not only look unsightly, but they also offer little resistance to passage of air, water and heat. For this reason, exterior work can become quite costly if old, obsolete windows need replacing.

Dealing creatively with interiors can be complex. Sensitive judgment, diligent engineering and fastidious construction are essential. Structural framing must be respected and sometimes reinforced. Upgraded plumbing and electrical wiring are always desirable and often necessary. Parts of the flooring may need replacing. And if kitchens and bathrooms have not been modernized for a half century or more, they may cry for a complete makeover.

During construction and in particular interior demolition, encountering conditions not seen during the initial survey is always a risk. Concealed, unforeseen problems — wood rot, mold, termite damage, failing structural connections, broken structural members, toxic substances — can result in project delays, unwanted design changes and additional cost. This is why it’s always advisable to include a generous contingency amount in the budget.

In addition to mustering sufficient financial resources for acquiring, restoring and renovating a historic house, you will need to muster non-economic, personal resources: flexibility, understanding, persistence, negotiating skills, endurance — and, above all, great patience. No matter how much time you hope and believe it will take to complete the work, it will take longer.

Roger K. Lewis is a practicing architect and a professor emeritus of architecture at the University of Maryland.