You can visit the unfinished Sagrada Família for €26 per adult through the basic ticket page. Your ticket includes the Basilica and museum with the audioguide app. The museum explains the building's design. Towers and the crypt are excluded.
The €36 Sagrada Família with towers ticket on the official tower ticket page adds one Nativity façade or Passion façade tower. The Tower of Jesus Christ is excluded. You take the lift up and walk down. Children under 6 cannot visit the towers.
The Basilica's FAQ gives a sale window of roughly two months before your Sagrada Família visit. Buy through the official website or app. If your date has sold out, check another slot or a tour with entry included. Bring the photo ID matching your ticket, as required by the Basilica's rules.
The Basilica's opening-hours page lists the visiting hours: In October, the Basilica opens 9:00–19:00 Monday to Friday, 9:00–18:00 on Saturday and 10:30–19:00 on Sunday. Arrive at the Nativity façade entrance at your booked time; late arrivals cannot enter. The official conditions of sale rule out re-entry after you leave the grounds.
Your ticket: Sagrada Família, €26 per adult, for the Basilica and museum with the audioguide app.
Plan around: morning for the Nativity side's bluer glass; evening for the orange glass facing west.