Working with Assessor Data

Having worked in the realm of affordable housing and economic development for the last several years, I have learned that assessor data is both a necessity and a pain in the neck when it comes to sales information. For most purposes, MLS (multiple listing service) sales data is sufficient. However, given the level of detail required in, for example, a housing needs assessment, MLS will typically not suffice.

Some reasons not to rely solely on MLS sales data:

1. For Sale by Owner properties are not included.
2. Realtors can make mistakes in recording the data.
3. Sales prices tend to be inflated.
4. MLS data is proprietary and not always easily accessible.

Some reasons to use assessor data include (the necessity part):

1. It provides the actual sale price recorded on the deed.
2. It includes all sales, not just sales from the MLS.
3. It typically includes the last few sales for each property so you can see property appreciation over time.
4. It provides multiple classification fields.
5. It includes ownership information.  This is particularly helpful in areas with a large influence from second homeowners.  Coding the sales by the zip where the purchaser gets their property tax bill can shed light on changes in second homeowner trends.  This is important in projecting future housing needs because second homeowners decrease the supply of housing available for residents.

Some words of caution in working with assessor data (the pain in the neck part):

1. Each County has their own system of recording and maintaining their data.  Make sure you fully understand the codes (look for codes indicating improved residential, deed type including deed restricted, qualified sales, exempt properties etc.).  If something does not make sense, call the assessor.
2. All sales are recorded, which can include parking spaces, transfers of property between family members etc.  Be careful to look for the deed type and whether the sale is qualified or not.  Also know that most assessors classify mobile homes differently and can exclude them from residential sales.  Additionally, apartment properties are typically classified as commercial.
3. Some counties will provide a sales file but it will only represent the most recent sale of each property.  There should be duplicate account numbers in the file, especially if you are looking at a period of several years.  Other counties will provide the last ten sales per record instead of showing each sale as a record.  This will require some manipulation to separate each sale as an individual record.

Thankfully, assessors are consistent in their recording and don't change their systems often.  So once you have a method for working with a county's data, you can re-use it over and over again.

Note: If you live in a non-disclosure state, you will have to make due with MLS and local Realtor interviews.  Another improving data source is trulia's stats and trends section.