Both Zillow and RedFin will always have a chance to be inaccurate. Homeowners spend thousands to update their kitchen, baths, etc., which surely will not be considered in those estimates. There are other factors to consider including the overall condition of the house – inside and out. The best estimate comes from a person who will go visit the house and then evaluate.
Source: Zillow vs. RedFin: Which home appraisals are less wrong?