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What is The chances of having an unaffected baby, when both parents have achondroplasia, are – and explain in detail?
First see general features of autosomal dominant disorders -

Autosomal dominant disorders are manifested in heterozygous state.
So, at least one of the parent should be affected to transmit the disease to child.
Homozygous state does not exist as it is not compitable with life, i.e. homozygous fetus usually dies in utero.
So, all the affected individuals will be heterozygous.
If an affected person marries an unaffected one, i.e. if one of the parents is affected than every child has 50% of chance of having the disease and 50% of chance not having the disease.