Sagrada Familia Tours

I was there less than 3 weeks ago.
We purchased tickets 10 min before the selected time slot because we weren't sure about the time we will be there. Walked to the first security guy, he checked the e-tickets and we walked into the security line. Then we were free to roam and proceed with our visit.
No need to be there 30 min before your time slot, they won't let you in anyway.
I suggest booking the tickets in advance though if you're planning to go in spring/summer. I checked availability for a few days before making the decision to buy them once we were there.
I also suggest using the free audioguide. It works very well.

There was a very long line at the church when I walked past it one May afternoon in 2019. I would not recommend showing up without a ticket, but it's difficult to predict visitor levels at the moment. It's possible you'd luck out.

Tickets are only available online. There are no lines to buy tickets at the Sagrada.
We bought them online ten minutes before our chosen time slot and there was a lot of availability. But that was the end of February.
In the warmer months I suppose that's not always the case. If you already have a set date in mind, I suggest buying tickets at least a couple of weeks earlier.

My family did the Gothic Quarter, Guadi and the Dark past-night-walking-tour by RunnerBean Tours in May of 2018. All excellent tours. Would highly recommend.

Thank you! Great feedback! My only question - and I know I sound obtuse (sorry!) - but if it is no problem getting in as long as you are not late and buy your ticket online, then what is the issue with the "long lines" that keep you from getting into the church that I keep reading about everywhere?

I did the English tour in Sept. and found the guide to be very easy to understand and the tour to be excellent value. They used the whisper system so you can take photos, etc. while still listening. There were not multiple languages, just English.
There was no line in Sept. 2021. Covid has changed a lot of things like entry times, tours, and booking slots.

It's difficult to predict how long the number of tourists will remain low. It is, after all, only March. I saw some really low hotel rates for April and May a couple of months ago--not in Barcelona, but in other high-traffic cities like Rome and Venice. I booked a room in central Rome for under 50 euros per night. The cost of that room seems to have more than tripled. I think lots of people have decided to travel who were sitting on the fence (or at least not booking hotel rooms) back in January.

I'm really reluctant to assure folks traveling later this year they can wait to buy tickets after they arrive in Barcelona. I did that in 2016, but I was spending ten days in Barcelona, and I knew I could easily pick up tickets during that time. I don't know your plans, but lots of folks who ask questions about Barcelona on this forum only plan 3 or 4 nights in the city (which is only 2 or 3 full days). They really can't afford to find that tickets for everything are readily available, but only between 4 PM and 8 PM. Or only between 11 AM and 3 PM (when they really might like to leave time to eat lunch).

I think this year is not a great time to buy sightseeing tickets earlier than necessary--something I've never liked to do, even pre-COVID. I'm holding off myself as long as I dare (and a good thing, too, because I'm having to delay my trip). If you're willing to put in the time, you can do the circuit of all the websites every day to check on ticket availability for your travel period and the previous week. That should give you some warning of potential problems while you still have time to grab tickets.

@jndgottfried
There was a big queue when the ticket office was right at the Sagrada.
At the moment there are long lines on Sundays when it is free to enter the church (Sunday mass). There are a lot of people outside the church taking photos and such or waiting for their time slot, but no long lines for the visitor entrance.

So...jumping in to clarify (we will be there end of May), I do NOT need to allocate extra time for security because we enter at reserved time slot and THEN go through security. Correct?

@Melinda
That's correct.
I would be there just 10 minutes before your time slot to see if the security guard could kindly let you in a few minutes earlier.
Ps. Check their website closer to your travel dates. They might always add some new info regarding visits in the following weeks.

The audio guide was great and allowed us to visit on our own timeline rather than when the guide felt it was time for the group to move ahead. When we were there people were queued according to their reservation time, so no need to arrive early. We did purchase the first tickets of the day so the crowds were minimized.