Is well built hyphenated

It depends on how it is used. When it comes before a noun and acts as an adjective modifying that noun, it is hyphenated: We expect the holiday party to be a well-attended event.

However, when it is used with a linking verb and does not come before a noun (that is, when it is used as a predicate adjective), it is not hyphenated: The free outdoor concerts were typically well attended.