Hola Julie, To clarify your tenses:

The structure "Voy a construir" which is the verb Ir + a + the infinitive, is called El Futuro Próximo. It states what you are going to do in the near future.

The Present Continuous "I am building" would be "Estoy construyendo," and is the tense used to describe an action that is currently in progress i.e. happening now e.g. you are up a ladder and someone asks you what you are doing.

The present infinitive of Spanish verbs is very flexible so it is possible to say "Yo construyo una casa" [literally I build a house] and translate it as "I am building a house," in a situation such as when you meet a friend and are telling him or her about your plans. In that situation you could also say that you are "going to build a house" and use the Futuro Próximo "Voy a construir," but you couldn't use the present continuous "Estoy construyendo" for the reason given above. It's all down to what exactly you want to say in this particular situation.

I hope that helps. Saludos. John

thank you John

I appreciate your time answering my question and now I understand

Julie