Question: [This answer](https://space.stackexchange.com/a/27968/6836) says:

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> We do not have a human-rated rocket which can send a sizable payload to Mars. The Falcon Heavy is certainly capable of sending a payload there, but the rocket would need to be human-rated, which basically means tested far more extensively than it has been now.
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And that got me thinking - for a vehicle to launch from the surface of the Earth it needs to be pretty substantial. But you can avoid a lot of the requirements of the stresses of launch by becoming a payload (e.g. satellites wouldn't survive a launch if they weren't inside of a rocket. And if it were economical to build satellites that could be self-launching, they'd already exist, right?).

So would it be better to just build a ship in pieces and just send it up on a (couple) of Falcon Heavy Rocket(s)?

Answer: 
Better by what criteria? It's a tradeoff. Assembly from parts makes less efficient use of mass, due to the need for mating structures and a larger area of hull wrapped around smaller bits of volume, and greater complexity due to the need to design all the separate parts and assemble them in orbit.

A modular spacecraft is also unlikely to be able to withstand reentry and landing on Mars. This means that it will need enough propulsion to brake into Mars orbit on its own power, and then send a separate lander/return vehicle, which itself will have to operate as an independent spacecraft. SpaceX's BFR instead uses a direct entry and lands the entire spacecraft after using the atmosphere to remove almost all of the kinetic energy of the spacecraft:
<http://www.spacex.com/sites/spacex/files/making_life_multiplanetary-2017.pdf>

The modular approach is inefficient and complex, but it can be done with existing launch vehicles. SpaceX initially proposed the even larger ITS, but could not fund the development of a system that would find little use outside of Mars missions. BFR is that system scaled down to be useful as a commercial launcher on Earth.
As several people have said there are lots of options and lots of trade-offs. To give you an idea, [here](http://www.spacex.com/mars) is SpaceX's presentation of their ideas for Mars. To summarise.

1. BFR takes off from Earth and leaves BFS (upper stage) in orbit
2. Repeated (maybe 6) BFR launches take tanker-loads of fuel to the BFS and partly refill its tanks (it can hold about 1000 tons of propellant, but uses about 900 of them to get to orbit).
3. At some stage the crew go aboard. More launches maybe necessary to carry supplies for the crew.
4. Using the fuel that was carried aboard, the BFS flies to Mars, enters the atmosphere, and lands, Taking nine months or so.

After this there are two ways it could pan out:

5a The BFS is refueled with propellant made on Mars and stored there by earlier robot (or manned missions). It can take off, fly back to Earth, reenter and land. It might be wise to transfer the crew to a nice new BFS before reentry, in case theirs has taken damage on Mars or in space

5b A much smaller launch vehicle (carried to Mars as cargo) launches the crew to Mars orbit, where the rendezvous with yet another BFS that has been waiting, to take them back to Earth

5b leaves a ship stranded on Mars, but doesn't need propellant manufacture there. It also means you have much less cargo space, so your mission can do much less.

This isn't the only approach, but it one. Empty spacecraft are fairly light, so launching the spaceship in one piece and then launching fuel and cargo afterwards is quite appealing if you have enough launchers and the ability to send them up frequently