Cat flap has to go temporarily. litter tray or just let him in and out ?

[MaryIsA]

Cat has cat flap at mo. Building work means we are losing back door and cat flap for 4 months.

Nowhere else to put one. We are going to have to let him in and out of front door. We are going to have to have a litter tray aren’t we?

Yes you are!

Of course you are. You should always have a litter tray anyway, even if you have a flap. Cats often don't like to go out when it's raining, or the ground is frozen, or there's another scary looking cat or fox outside. It's also a way you can quickly realise that he has a bladder blockage (as he will be constantly going to the tray but not doing anything) and that could save his life.