Peter's Basilica, men and women are required to cover their knees and shoulders. Men can wear trousers and shirts, while women can wear long skirts or dresses, both of which must cover the knees.
Can I go in shorts to Sagrada Familia?
However, in order to ensure a smooth visit please observe the following Sagrada Familia dress code rules: Tops must cover the shoulders. No really low necklines or see-through clothing. Shorts and skirts must go down to the knee.