Do residential property values go up when an area becomes a Historic District?

I have often heard that the high Historic Design Review fees in Portland are offset by higher property values within Historic Districts.   Irvington became a Historic District in October 2010.   Did becoming a Historic District increase the sale price of residences in Irvington?  

The chart below shows the average sale price per square for residences within the Irvington Historic District as a percent price premium over the rest of 97212.

Irvington Price Premium

The data clearly shows that the creation of the Historic District has not affected property values significantly.   As expected with a relatively small data set, there is some yearly fluctuation.   From 2004 on, we can see the price premium varies about 6% per year.   The following chart shows the number of residential sales in 97212, where sales in the Historic District average 31% of the total.

97212 Sales per year