If one parent is average height and the other parent has achondroplasia, is that the dominant trait meaning all of their children will have achondroplasia also?

The gene associated with achondroplasia is inherited autosomal dominantly, in other words it does not skip a generation. It can also occur sporadically. A consult to a genetic Specialist may help with family planning. More information: https://www.lpaonline.org/assets/documents/NH%20achondroplasia.pdf

The pairing of a normal with an affected person has a 50/50 chance of producing either normal or affected offspring. The affected parent passes either their normal or their achondroplasia gene, Dominant means it only takes one of the achondroplasia to produce the condition. Anytime 2 achondroplasia genes occur in a fetus the condition is fatal so all affected people have only one of these genes.