I have flooding in my garage for which I cannot readily work out why it is happening.

I have noticed that there is overflow from high guttering but it does not flow in that direction.

I have also noticed that the guttering in the vicinity is in four levels
1. main house highest
2. Garage
3. 2nd level house
4. Carport

There was an addition to the house about ten years ago which increased the catchment on the 2nd level by about 140 sq feet

All these feed into one storm water outlet which as far as I know is not blocked. The fall on the property is from the rear of the garage to the street and away from the garage towards the house

Any suggestions would be welcome

Does it flood only when it rains ordo you suspect it might be a plumbing issue from the house?

Thanks for your response
It only floods after rain but as it doesn't readily dry out it is difficult to tell after frequent rain. The garage is not attached to the house, it is separated by a small gap ( path )between the buildings and no water pipes are close by

Several possible sources of the water come to mind.
1. Rain water as in a leaking roof.
2. Rain water run off as rain water in gutters and down spouts.
3. Ground water run off as in both rain that fell on the ground and rain water from the gutter and down spout system.
4. Under ground water as in an under ground spring under the garage floor.
5. Busted or ruptured pipe.

Since you say there are no pipes in or under the garage, I think you can eliminate that as a real possibility.
Since a garage floor is usually at pretty much grade level, unless you live in a hole like I do, I think you could eliminate under ground water. While water from an under ground spring might only appear after a rain, it would probably appear a little while after the rain started and would continue for some time after the rain has stopped.
Ground water run off, both the rain that fell directly on the ground, and the run off from the gutters and down spouts should be visible and you should see it running into the garage.

A leaking roof should be visible or the damage to the ceiling from a leaking roof should be visible.

That leaves gutters and down spouts. It's really hard to envision your guttering system since you say the garage is detached from the house and separated by a path. Assuming the garage has gutters and down spouts. Also assuming that the house gutters are not attach to the garage gutters, but then maybe they are.. You should be aware that stopped up gutters can overflow the back side of the gutter and cause water to run inside the structure. If the inside walls of the garage are finished (like drywalled) that water can flow down the inside of the wall and exit at the floor. Normally you should see damage to the inside wall surface if this happens repeatedly. All depends on how the inside walls are finished. If a plastic moisture barrier was used, or if the foiled faced insulation was used, you might not see damage to the inside surface.
Since you said;

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I have noticed that there is overflow from high guttering

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I would say you probably have stopped guttering or down spouts there. Therefore, I would also say check you garage gutters and down spouts. Take special care to insure that the down spouts are clear.

Thanks again

Yes the possibility of a blockage in a down pipe exists, that is recent and the storm water to the street appears to flow strongly. We clean the gutters and check flow about each six months

What I am dealing with here is I find water on one side of the garage to about half the width after rain, apparently the floor is not entirely level and water can accumulate to a depth of half an inch. This floods the floor to about half the surface area and means the garage cannot be used as a workshop. There is not enough buildup for the water to flow out the garage door.

My thoughts were that as parts of the system, the garage, car port and house storm water all feed one outlet pipe that a back up would cause the garage guttering to overflow. I expect the same is true if the outlet pipe is inadequate. The garage walls are constructed of cement block and do not appear to be wet so I can find no evidence of gutter overflow getting inside the building. There is no evidence of water coming through the roof. Do you think it possible that an overflow could run down inside the blocks and the blocks act as a sort of reservour.The fall on the ground is such that a gutter overflow would flow away from the building. There is no evidence of water entering via the garage door as the area immediately behind the garage door is dry Your theory about a spring is interesting but as the floor is a concrete slab I have discounted that. I have long though that it maybe general storm water flowing from other properties but I cannot find where it enters the building

Yes, water can easily flow down the cavities inside cement blocks. When it reaches the concrete pad or foundation it has no where to go except out through the gaps or cracks in the mortar. Punch a small hole in a block about an inch above the floor and watch the water run out.

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we clean the gutters and check flow about each six months

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Every time I clean mine more leaves fall before the next rain.

Hi thought you might like to know the outcome

Drilling the walls of the garage yeiled no result however we decided to drill right though the wall and create a weephole so the water is draining from the garage floor. Now that the rain appears to have stopped we will investigate putting a drain at the rear of the garage as there is indication of water from other properties. Our investigation of the down pipe found not only blockage but also a corroded down pipe which became a big hole when touched, however no clear answer as to where the water comes from. Thanks for your help