The following is a simple copy/paste from DCAD's Residential Building Classifications PDF
Class 1 is a very basic house. It is frame construction on post foundation. It is substandard housing.Roofing is composition shingle or roll composition. These houses have no insulation.
Class 2 is on post foundation with wood siding exterior and comp single. It meets minimum property standards. There are deviations for pier and beam foundation, wood shingle roof, etc. This class of house was built as a rule before 1945. They usually do not have attached garages.
Class 3 is a frame dwelling on a slab foundation. It meets minimum property standards. It generally has comp shingle roofing.
Class 4 is a frame structure on pier and beam foundation. It exceeds minimum property standards. It generally has comp shingle roofing. They may or may not have attached garages and were generally built after 1945. There are many deviations with this class of house; siding, roofing, and foundation, heating, A/C, etc.
Class 5 is good quality frame dwelling on concrete pier and beam foundation. They typically have comp shingle roofing. There are many possible deviations for this class.
Class 6 is very good quality frame house on a concrete pier and beam foundation. It is similar to a class 5but much better quality.
Class 7 is excellent quality frame house. It is the top class for a frame siding house.
Class 8 is an old style brick veneer house built on a pier and beam foundation. It meets minimum property standards. It has comp shingle roofing. There are a few deviations such as wood shingle roof, stone veneer, etc.
Class 9 is an old style brick veneer house which is of good quality. Similar to class 8 in age but better quality and more gingerbread. The classic class 9 house has a steep gable roof. It is built on concrete pier and beam foundation. There are several deviations for the various components.
Class 10 is a old style, very good quality brick veneer house built on a concrete pier and beam foundation.They typically have clay tile roofing. There are several deviations such as exterior walls can be stone veneer.
Class 11 is a solid masonry dwelling on a concrete slab foundation. It meets minimum standards.
Class 12 is a brick veneer on a concrete pier and beam foundation. It slightly exceeds minimum property standards. It is similar to a class 8 but is not old style.
Class 13 is a brick veneer house on a concrete pier and beam foundation. It is average quality. It typically has comp shingles. Most commonly built from about 1945 through the mid 1960’s.
Class 14 is brick veneer on a concrete slab foundation. It is of average quality and typically has a comp shingle roof. Tract homes are class 14’s. They typically have 2 bathrooms. They were built in the early1960’s to present date.
Class 15 is a good quality brick veneer house. They are similar to a class 13 but are better quality. They are on pier and beam foundations. They were built in the late 1940’s to the mid 1960’s.
Class 16 is a brick veneer on a concrete slab. It is a better than average construction house. It is a better quality class 14.
Class 17 (so such class in DCAD's system)
Class 18 is a brick veneer good quality house. It is a step up from the class 16 house. It is a new version of a class 15. It is typically a comp single roof house built on a slab foundation. It has a deviation for a pier and beam foundation.
Class 19-20 (so such class in DCAD's system)
Class 21 is a very good quality custom brick veneer house. It is better quality than a class 15 or class 18. It will have wood shingle or better quality comp shingle roof. 3 or more baths is the norm. It can be on a slab or pier and beam foundation.
Class 22 (so such class in DCAD's system)
Class 23 is a superior quality custom house. It will have 4 or more bathrooms. It will have custom touches throughout the house. IT will typically have a large garage.
Class 24 is an excellent quality, elaborate brick veneer house. Everything about this house is top grade. It is a step above a class 23.
Class 25 is an excellent quality older style brick veneer house. Typically built before 1950. In it’s day and time it was about the best house there was.
Class 26 is the absolute best house made. They are of the finest quality and most elaborate finish out that there is. They are unique and extremely custom from start to finish. They are estate properties. They have features such as heated floors, tennis courts, materials made special for only this house, imported specialty items, etc.