Most listings are in the $20's per square foot per year. The Walnut region is going through a an increase in rates. In the last 3 months, the median rate increased $0 per sqft per year. That's a 1% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Walnut region has less expensive rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 3 listings have come on the market, which means leases might be leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 800 sqft. Most properties in this region are approximately 1 floors. Most buildings in this market were built in 1987.