It explains on the NA home page what they all mean!!
Classification of buildings as published in 1839
HOUSES AND OFFICES
New or nearly new
A+
Built or ornamented with cut stone, and of superior solidity and finish
A
Very substantial building, and finished without cut stone ornament
A-
Ordinary building and finish or either of the above when built 20 or 25 years
Medium
B+
Medium (not new), but in sound order and good repair
B
Medium, slightly decayed, but in good repair
B-
Medium, deteriorated by age and not in perfect repair
Old
C+
Old, but in repair
C
Old and out of repair
C-
Old and dilapidated, scarcely habitable
DESCRIPTION OF HOUSES
1
Includes all slated dwelling house built with stone or brick and lime mortar
2
Thatched house built with stone or brick and lime mortar
3
Thatched house with stone walls with mud or puddle mortar; dry stone walls pointed or mud walls of the best kind
4
Basement stories of slated houses, used as dwellings
DESCRIPTION OF OFFICES
1
Includes all slated offices, built with stone or brick walls, with good lime mortar
2
Thatched offices built with stone or brick walls, and lime mortar
3
Thatched office, having stone walls, with mud or puddle mortar, dry stone walls pointed, or good mud walls
4
Thatched offices built with dry stone walls
5
Basement stories or cellars used as stores
Classification of buildings as published in 1853
CLASSIFICATION OF BUILDINGS WITH REFERENCE TO THEIR SOLIDITY
1
Slated house or office built with stone or brick and lime mortar
2
Thatched house or office built with stone or brick and lime mortar
3
Thatched house or office with dry stone walls with mud mortar, with dry stone walls pointed or with good mud walls
4
Basement of 1
5
Office with dry stone walls
CLASSIFICATION OF BUILDINGS WITH REFERENCE TO AGE AND REPAIR
New or nearly new
A+
Built or ornamented with cut stone, and of superior solidity and finish
A
Very substantial building, and finished without cut stone ornament
A-
Ordinary building and finish or either of the above when built 20 years
Medium
B+
Not new, but in sound order and good repair
B
Slightly decayed, but in good repair
B-
Deteriorated by age and not in perfect repair
Old
C+
Old, but in repair
C
Old and out of repair
C-
Old and dilapidated, scarcely habitable