Basic objectives to use in your decision-making tree for Preservation.
If it was always there, don't remove it.
If it was never there, don't add it.
Conjectural elements are not appropriate. Don't try to make your house "more historic" by adding elements that you felt the original owner would have added if possible. Rely on photographs from the past, on the "shadows" that the removed elements may have left, or from careful research of the original architect's known works.
When working on the historic elements of your home, only replace what absolutely can't be repaired. Always choose the most conservative methods possible. The order of preference is:
Maintain - don't allow historic elements to decay. Clean, paint, patch, fix leaks.
Repair - if there is damage, consider repairing what is there rather than jumping directly to replacement. For example, often windows can be repaired even if they appear beyond redemption. Siding can be pieced-in rather than stuccoed over. Save money, conserve materials and retain the value of your historic home.
Replace - only when there is no alternative, and only with elements as close as possible in materials, design and workmanship. Contact Planning for resources and advice, as well as approval. Replace with replications of the original, including wooden windows with identical design details, doors that are not "fake old" or inappropriately over-decorative, scored concrete that matches the original and has a smooth wood float or rag-wiped finish - no "broom-finish." Replicate the original.
A word about windows: many of our new neighbors are coming before the Board to ask permission to REMOVE their vinyl windows and restore them with wood. Installing vinyl reduces the value of an historic home a great deal. Moreover, window salesmen use "energy efficiency" as a selling point, when a set of attractive window coverings does more, and costs less. And after all - we are in Los Angeles, not Minnesota or Arizona. And remember - changing anything on the exterior of a home in an HPOZ requires approval; you can be fined and required to remove windows installed without HPOZ review, and to restore the originals.