Thank you for your enquiry.
To answer your questions. Yes you do need plans for a Wendy house/store room and they need to be submitted to the municipality for approval. I would however suggest that you call it a "Garden Shed" or a "Tool Shed" rather than a store room, workshop or a Wendy house because a store room will bring in certain Health and Safety regulations that you don't want to be involved with. A Wendy house or workshop suggests that it will be occupied on occasion and ventilation will become an issue and a window will be necessary. If it is a Garden or Tool Shed then a window is not required.

The shed can be built against your boundary wall provided that the roof does not go beyond your boundary and that there is a gutter that will bring any rainwater back into your property. If it is built outside the permitted building line for your property which is usually 3m from the rear boundary and between 1m and 1,5 m from your side boundary depending on your local regulations, (ie, against your property boundary) then you will need your neighbour's permission (usually all of them).
If it is submitted as a Garden Shed then windows are not required although I would recommend air vents of some kind otherwise condensation inside the store will create mould if there is no air flow. I have not been able to find any regulation limiting the size of the unit with particular reference to ventilation but I would suspect that once it went beyond a certain size, a window may be required and certainly fire regulations may come into play.
With regard to raising your Vibracete wall:-

There are restrictions on how high boundary walls can be, usually 1,8m but once again with neighbour permission and with good motivation (security issues are always a good one) you can go up to 2,1m or even 2,4m without difficulty. Just make sure that the existing wall is in good condition because wind forces on a 2,4m high wall are a lot greater than for a 1,8m. Plans are not required for side boundary walls if the height of the wall remains within the local council height regulations. Once you go over that, then it may become an issue as it starts involving your neighbours.
Your local municipality will be able to confirm what I have just said.
I hope that this helps.