Is planning needed for putting a prefab garage on the side of my house?

All councils can be different with regards to their allowance and wiggle room, it may be worth a call to speak to a human ( rare I know), but I've built a log cabin in a garden near hear that was a living space and it was completely legal due to it being a single story and essentially a big shed, therefore a temporary building so to speak.

Yes..you need permission from the counsel and autorización for building

Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
Maximum height of 2.5 metres in the case of a building, enclosure or container within 2 metres of a boundary of the curtilage of the dwelling or a house.
No more than half the area of land around the “original house”* would be covered by additions or other buildings.

Anything more than 15 square metres then you will need permission.

You can build a garage or outbuilding on your property without planning permission as long as it's of a reasonable size – no higher than 4 metres. Do bear in mind, though, that outbuildings can not take up more than half of the land around the original property.

depends how high the garage is but generally if its under 8foot then no

Shouldnt need planning as it would come under permitted development as long the max height is no more then 2.5 mtrs

You can build a concrete sectional garage or a prefabricated garage without planning permission as long as the garage doesn't take up more than half of the land around the original property.
Another aspect you have to consider is the height. Any garage higher than a single storey you need permission. you can’t go over 4meters high either so if you have a pitched roof on your single storey garage the total height with the apex can’t go over 4 meters.

Another aspect - you cannot have a balcony, veranda or raised platform! Around your garage.
Another point to check is your rights on the property! some properties if you live on a housing estate say you have to keep the space as a parking area if this is so you need permission aswell.
Finally Make sure you don’t use the garage for domestic purposes and you will be all good!
KLERA

I've just worked with a neighbour building an extension to the rear of their house and they didn't require planning permission.

It has to be under a certain height from memory.

Also, I don't believe the previous owner of my house had to get planning permission for the prefab garage we have in our back garden.

Yes you need planning per mission if it’s higher than 2 meters