How St Peter's Basilica was built, architect by architect

When someone asks me who built St Peter’s Basilica, the honest answer is that a whole century did. Work began in 1506 and it was consecrated in 1626. Twenty popes and more than ten architects came and went, and almost every one of them changed the previous design. Grasping that is what turns the visit into more than a parade of marble. Here’s who did what, what shape the church is and what the official figures say, which don’t always agree.

One basilica on top of another

There was an earlier one. Around AD 324 Constantine began a great church with five aisles, a transept and an apse over Peter’s grave, which sat at its centre. In front were a flight of steps and a four-sided courtyard. To build it he buried an entire Roman cemetery, the one you can now visit by booking, beneath the floor.

That basilica lasted over a thousand years. Julius II resolved to replace it early in the 1500s, and building began on 18 April 1506 at the spot now occupied by the pier of St Veronica. Much of what survives of the old floor level is the Vatican Grottoes, which are free to walk through.

Who designed St Peter’s Basilica?

Lots of people, taking turns. The Fabbrica di San Pietro’s official site lists the architects in order. Fra Giovanni Giocondo, Raphael, Giuliano da Sangallo, Antonio da Sangallo, Baldassarre Peruzzi, Michelangelo, Jacopo Barozzi da Vignola, Giacomo della Porta and Carlo Maderno. Its FAQ page sums it up with four other names, Bramante, Raphael, Michelangelo and Bernini.

I find it more useful to look at what you can actually see of each of them. Of Bramante and the early directors almost nothing recognisable remains, because their designs were replaced. Michelangelo gave it the dome. Maderno, the long nave and the facade. And Bernini, who isn’t on the official list of architects, created the interior you’ll remember, from the baldachin to the Chair of St Peter, plus the square. That’s why the ranking above doesn’t measure genius. It measures what you’ll find.

The dome, from Michelangelo to Della Porta

Michelangelo designed it from late 1546, under Paul III, and when he died in 1564 the work had only reached the drum. Giacomo della Porta and Domenico Fontana picked it up under Sixtus V and closed it in May 1590. According to the Fabbrica, the vault was finished in 22 months with 800 workers. On 18 November 1593 the gilded bronze ball and cross went up on top.

The Holy See’s fact sheet gives the measurements. The dome is 41.50 metres across inside and 58.90 outside, weighs 14,000 tonnes and reaches 117.57 metres to the vault of the lantern. It has a double shell. The inner shell carries the load and the outer one, clad in lead sheets, protects it. The route up runs between the two, so climbing St Peter’s dome is also the best architecture lesson in Rome.

What shape is the floor plan?

A long central nave crossed by a transept, with the dome directly over the crossing, above the papal altar, which in turn sits over the apostle’s tomb. The whole building revolves around that point. What changed the plan was the early 17th-century decision to lengthen the nave by two bays, Maderno’s work, after which he built the facade of St Peter’s between 1607 and 1614. That’s why you walk for a good while before reaching the baldachin.

Be careful with overall figures. For length the Fabbrica says roughly 187 metres; for height, its FAQ says 136 and the Holy See says 133.30 to the cross. I’d rather give you both with their sources than invent an average. What they do agree on is that there are more than 40 altars and that about 91 popes are buried in the basilica and the Grottoes.

How to read the architecture on your visit

Here’s the route I’d take. Arrive at seven, when it opens and the nave is empty, and walk slowly down Maderno’s nave. Stop under the dome and look up. Carry on to the apse, where the Chair closes the axis. And, if there’s time and your legs allow, climb the dome to see how it’s put together.

If you fancy spending a little, the Octagonal Rooms inside one of the dome’s piers opened to the public for the first time with the Pétros ení digital experience, €22 including the basilica. And 2026 is a good year to go, as the basilica marks 400 years since its consecration with events that end on 18 November. For opening times and queues, see the main page on St Peter’s Basilica.