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you probably have a hatch that you push to lift out of the way when you want access.
Yes!
Often you convert that to one on hinges which flaps down so the ladder can be pulled down with a stick and hook.
Yes!
The recessed 'hatch' is 730x730mm from what I remember (will measure again) and I have plenty space for a 3 piece ladder in the loft too.
http://www.ladderstore.com/loft-ladder-range/loft-hatches/bespoke-loft-access-hatch.html
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That's some pretty fancy hatches they've got there. Nothing wrong with that. If you're providing that to a carpenter it shouldn't take too long fit, as he's said. You would need a standard A frame ladder to fit the hatch like that though as you're not going to able to fit it with the loft ladder extended, well you could but it would be an unholy pain in the ass.
Why not buy a 5 step aluminium A frame ladder, every house should have at least a 3 step one, if you have taller ceilings a 5 step is just as good, You can pick up an Aluminium 5 step one from Homebase for £30. The pressed aluminium ones are light and easy to move when extended, the 'safety ladders' are usually heavy and expensive, you won't want to decorate a room with one. You can store it in the loft once you've fitted the ladder!
Don't forget, ladders are one of the most common causes of diy accidents.
You could probably get a away with using the ladder you are installing to get into the loft, especially if you have a second person to foot it (stand on the bottom and make sure the foot of the ladder doesn't move). Then pull it up and fix the mechanism that holds it up there in the loft. Most people struggle to convert the loft hatch to a type that flaps down on a catch, you probably have a hatch that you push to lift out of the way when you want access. Often you convert that to one on hinges which flaps down so the ladder can be pulled down with a stick and hook. There are lots of types though.