If you are certain about going back to Sydney in couple of years - please do not waste the £10,000 by doing anything structural
I would pay more for a house that has potential than one that has been done wrong (not to my needs or desires or the way I want to live or use the space)
Yes deffo get planning permission for a double height extension to increase bedroom sizes and to convert the conservatory into a useable space 365 and 24/7 although obviously not 24/7 iyswim
I was confused in the photo about a huge TV on the wall, I now know they are old photos but if it is sunny how did the old people watch TV?
Also loft conversion planning permission if you want - especially if your bedrooms are small and others have it in the street
Also get price breakdown from several builders, (explain to the builder why he is quoting cos if they are busy they might over inflate prices) but in bits, like this room would cost this and then when you sell if people ask you can tell them how much the plans would cost ( use the cheapest quotes obviously)
I would myself prefer a solid walled and ceiling/roof room with velux Windows and large windows with thick curtains and/or blinds rather than conservatory, due to heat loss/gain and light control
So the thing is that different people live in a house differently - the one we live in now for example - it had a double length garage and a conservatory in garden, we pulled down the conservatory,
Out of all of that we built a bedroom, wet room, utility room and TV room plus huge extension to the sitting room, dining room and kitchen in place of their conservatory
The thing is, our house came with huge potential to extend to what we wanted so we paid more for it cos we wanted to make our choices rather than have somebody else decide for us, what to build where etc
One house round the corner had been extended BUT they had the downstairs bathroom at the front and the bedroom at the back so completely wrong for the way we wanted to live - you had to walk thru utility and past kitchen to get from bedroom to bathroom
So get planning permission done and decorate your DD bedroom and save the £10,000 to convert to $$$$$