oops with the diagrams.. Note that the "shed like" part of the extension joins onto what must have been the original lean to that extends all along the original villa part. Note the pillar in the living area, that is structural. Another strange aspect is that the toilet juts out into the open plan space ( I think that could be remedied with $$s and it could fit into the main part of the family bathroom by taking out the old corner spa and putting in a smaller freestanding bath for example) . By the way, the "retreat" is actually a courtyard that could be a nice feature if properly landscaped etc. A new roof (to get a new roof line) might work on the shed like part, but the walls around the west and southern perimeters of the space are just windows/doors so I don't think they could support the weight of a "proper" roof rather than glass? Can you see that plan well enough and what info do you still need OkLouise (and others)? Thanks again for any ideas :)

It depends on many factors, some interrelated, some not. Unless you are dealing with more off the shelf/volume/standardised design solutions I'd be somewhat wary of utilising any fixed overall /m2 calculations to base your decisions on what you can or can't achieve or should/shouldn't do for your project :) We prefer to present/offer our professional opinions of likely construction cost by collating the projected likely trade breakdowns carefully considered for each particular scenario proposed, factoring in various aspects such as quality, site, scale etc that will ultimately contribute to influencing the number involved. Have found this to be a more accurate/realistic approach than simply applying $$/m2

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....

Extensions are expensive. I can understand why you would be considering one if you had an existing house and needed more space, but it is poor economy to buy a house to then spend an extra $300K+ on an extension. Put your $300,000 into buying a more expensive and larger property that provides the accommodation you need will give you better bang for your buck, and you can see what you are getting. I regularly do pre-purchase design inspections to assess the viability of renovations and extensions for clients prior to them deciding to buy a property. This service is like a mini-feasibility study, and includes local government planning issues as part of my Dr Retro House Call service. For instance I had an engagement last week to inspect a heritage-listed 1960's ambulance station in suburban Melbourne to see if it was viable to renovate and subdivide into two residences. I managed to cover a lot of complex planning, design and regulation issues in a two-hour consultation to help the potential buyer make a decision. While it was my first ambulance station, I have completed similar pre-purchase design inspections for lots of more traditional houses. Best of luck with your considerations, and please contact me if you would like to book in a Dr Retro pre-purchase design consultation. Regards Dr Retro of Dr Retro House Calls