My $0.02 - in NSW designers are NOT registered or accredited, beware. All architects are nationally accredited. I suggest that there are a lot of things you can do yourself before you start - get a proper survey of your block, ensure that you get them to provide angles for anything that isn't square....understand if there are utilities like sewer pipes which will affect your setbacks or useable space...... Go online and workout the sun angles for your longitude and latitude helpful for solar and overall layouts. West facing picture windows are not the best. Some of the solar firms have online calculators which help to do this. Read the local council rules and understand them, apply them to your survey and your design ideas....including if you are a corner block and if you want to go two storey...... If it is still a 'go' Get all of the heritage, flood, fire etc checks done by council - around $150 depending on council........ Eliminate or at least understand all the big constraints before you go any further........ Look again at the survey- any contour lines may cost you money, assume a cost of at least $2000 per sqm to start and do some layouts of your ideas on paper, add up all of the spaces, can you actually put them on your block? How much is the total?.......yes garages are slightly cheaper to build but still take up precious footprint space.... Is access likely to cause issues for trades, equipment and deliveries?......... Keep refining and updating the ideas and plans thinking about how you want to use the spaces and move around inside and out.....where are you putting the wheelie bins, clothes dryer etc..... Try the free to use BASIX site forms if you are in NSW, understand how the size, shading and orientation of windows affects your sustainability and adjust, this will also apply many of the local council rules as well..... Do you want a brick house, with render? A pole home, a kit build? A fast build insulated panel build? What type of roof?...... The more you have worked out for yourself the better, look at the kit home options, there are lots and some are very good... Everything is delivered and you just construct with the trades and some may have builders in your area.....The new InsulLiving Code Mark BCA approved options are also worth a look..... Anyway do all of that before you line up for the fleecing floor lol

HI All - for some reason my lengthy reply a few minutes ago was deleted :-(. I put so much thought into it! So here it goes again. Basically a big thank you to you Houzzers. MB your subfloor ventilation sounds great and Im glad it's fixing the problem so simply. The reason why Im trying for a whirligig solution is that I favour a passive system, however if it doesn't work I'll look into what you suggest Siriuskey - yes to some kind of kit. I've already contacted a guy on Gumtree selling grannykit frames - he says for something to my specifications would be about $4000 for the steel frame alone. Would need to check engineering details - and the liveable sheds you've linked me to seem to have that all checked. Although getting a local Chippie to frame it all up on site - as Twos Co suggests - is probably the most economical option Two's Co - thanks for the feedback and the excellent checklist of processes. I've covered most of the concerns though - I've got the DA based on my own flimsy plans. Because it's relatively small, I was going to work with the builder directly and over-engineer it. I've since engaged an architect to do some handsketches and draft specs, which will be signed off by a structural engineer. It's basically a portal frame independent structure, with a 3 degree pitch skillion roof. The house was rewired and the wires you can see are non-live looms waiting for the extension! The sewer is not connected and that part was never plumbed in. There's good rear lane access, but for the purposes of getting concrete in, I'll most likely get a concrete pump from the front of the house - for $180 per hour they will save time and make the concreting much easier. The existing trench I got dug in because I had an excavator on site installing those lovely large concrete blocks you can see, for the retaining wall (500kg each). But that was 7 weeks ago, and the excavation has got too large, and I worked out for a very similar cost I can make the extension 1m wider, so why not. The existing trench I can fill in with the demolition rubble, compact, and filled with any fill waste. Two's Company, I'm on the fence about removing the existing Bedroom 3. If I thought it wouldn't add to the costs too much, I'd do it. One advantage is that I could then make the whole 9m long extension (Option 2) around 150 - 300mm higher, (ie a step up from the existing house) and thus avoid too much excavation and a potential pond. To answer your question about damp and moisture, the whole house is facing a hill with 7 degree slope, and fixing this has been the main aim of the retaining wall, however the runoff is only likely to increase in coming years. anyway here are a few photos. The site is still so messy because the demolition only happened 2 days ago - Im still tidying up....

Standard stumps required. You will need engineer to do testing and design. Want get a building permit without it. Talk to a local one who is familiar with conditions

Basically , you are paying professionals to strip out 80% of everything , so $1000's , then other professionals to strip out wiring and plumbing , to end up with parts of a 50 or 70 year old frame , that suited a 2 bedroom layout but probably not a 4 bed ( that's what I'd aim for by the sounds of it ) . I'd suspect it would be easy to spend $50k to end up with a compromise ( think doors not exactly where you want , window holes that are too little or high , room sizes too small ) . And then you'll find a builder will only 'discount' you $5k for what is there , and then charge you $10k for all the mucking around . If it was adding on a couple of bedrooms and a bathroom , certainly worth costing it ; if swapping a kitchen and a bathroom , probably ; but when 80% of the rooms , which means 80% of the floors ; 95 or 100% of the roof ; every external wall and 90% of internal ones -- ask why ? If you had a 20 year old car that was a bit dinged , but its done you well , you'd probably go to the panelbeaters and spend $1k , maybe $2k . But if it was a bit dented , rusty around the bottom , motor smoky , gearbox slipping , paint faded , headlining fallen off and both front seats ripped and torn -- you'd head to the car sales wouldn't you ? Or if you had no $$ , you'd hope it lasts another 6 months . Most people wouldn't spent $20k or $30k on it unless it was extremely special .