The error:

• The property description says you have a 600 square foot basement suite.  However, you do not have a suite.  Your basement is completely unfinished.
• You find out from BC Assessment that the basement suite is valued at $25,000.
• Your total property assessment is $350,000.
• This error is equal to 7% ($25,000 divided by $350,000).
   

Is this a "large" error compared to your assessment?

A 7% error is large enough to request a change in your assessment.  Usually, we say:

• more than 5% is significant (or large enough to request a change);
• less than 5% is too small to request a change in your assessment.

In this example, the answer to the question would be "yes".  The error does make a large difference.