There is nothing wrong with having the board for the pelmet sitting on top of your window architrave. I can't tell much from the photo you have provided, but how an entire wall of curtain would look would really depend on the colour of the curtain. If you had navy for example, like you have now, would make the wall similar to a feature wall and would affect the visual proportion of the room. However, if you had light coloured walls and a pale coloured curtain then the wall of curtain will add to the lush look to the room without visually altering it.

What is allowed will depend on your council, and I agree that developers of new estates seem to get away with everything. I queried my council about this and they said that only "non-living"spaces (I can't remember the term they used), can be built to the boundary fence. They said cupboards don't fit into this category - only things such as garages. They advised that it wouldn't probably be approved, but if I wanted to apply for a relaxation, then I should get my neighbour to state that they are okay with what I wanted to do and submit that with my application. Otherwise, how close you can go to the boundary is dependent on the height of your house. If lower level, you can go close to the boundary, then if the extension is double storey.

Why can't you just position the rod centrally over the window with a 15cm extra rod from window frame on either side for curtains to collect. With a central opening. dont worry about the off centre. And from your photos I would measure the wall from window to end of wall on each side. It is a minimal difference. Looks more like an optical illusion because wall where the person is sitting is angled. If there is a big space there, stick a nice tall plant there to even if up.

Take the top off ( display Cabinets on either side of T.V. , Stuck in the middle ). The lower horizonal long glass cabinet to keep and cupboards to re-top with likely laminate MDF , table top structure