So the garage doesn’t currently have a back to it, it doesn’t have a side either but that’s going to be sorted with some OSB as it’s <1ft from the neighbours house wall so security isn’t much of an issue form that side. The base is already a concrete slab that looks to be pretty thick (there’s a 1ft drop to the gravel below between our garage an the neighbours house).

The rear is an opening that presumably used to house a second garage door, about 2300mm x 2100mm (WxH) if you ignore the inner wooden frame. Which seems to be conveniently about 200mm smaller than most ‘patio door with windows either side’ double glazing units, of which there’s loads 2nd hand on ebay for peanuts after people build extensions.

How much trouble will this be to fit?

Plan is:
1 course of bricks to make sure it’s above any potential surface water on the slab outside the garage (it drains away from the wall, but just to be sure).

Remove the existing (painted softwood) garage door frame and secure a new correctly sized frame to the structural woodwork of the garage to which the double glazed unit will be attached.

Fit the frames, window units, doors, etc :D.

Stand back and bask in it’s glory.

Apply vast amounts of silicone sealent after the first time it rains and leaks like a sieve.

Googling throws up the usual range of opinions from don’t even try to just keep everything square and plumb and work round it methodically.

The alternative is another garage door, but that’s at least double the current budget and probably not as secure.

Security being relative. I take it you’re not that fussed about double glazing letting anyone see in then? (serious question)

It should be easy enough to fit as long as, again, everything is square and level. Take your time, do it right and it should be a simple job, it’s how these units are usually fitted anyway.

The OSB side doesnt sound like a great idea IMO. Even exterior grade will get wet through eventually.

Security being relative. I take it you’re not that fussed about double glazing letting anyone see in then?

Not overly fussed, there’s a lock on the side gate and if you’re prepared to force that then you’re probably going to make a beeline for the garage anyway.

The OSB side doesn’t sound like a great idea IMO. Even exterior grade will get wet through eventually.

I plan to paint it, and the roof overhangs by about a foot so it only get’s wet when it’s really chucking it down and windy.