Underwriting determine expenses on 1930's apartment investment

[Scott M.]

All,

Looking at 33 units spread across 3 buildings in a historic area. All units have been renovated completely. Just curious in general I'm looking conservatively what % of income goes towards expenses usually on buildings of this era? I understand all the common stuff ( roofs, electrical, etc) but are there certain expenses that are greater with buildings this old compared to newer ( 20 yr or less) buildings? Also, how to determine if structure is sound and the life cycle of something like this? How much more expensive of replacement of parts to meet historic code?