I should add, we’re in regional Victoria (Australia)

The accuracy of a builders price depends on the information that he can price from. Any builder can given you a ballpark figure without any drawings or documentation but it will be very rubbery, so probably plus or minus $100K (usually plus). It is a waste of time at this early stage to be shopping around for other rubbery figures.

I am happy to give you a rubbery figure without even seeing your home and I would anticipate that you would expect to pay between $300K and $500K for two attic bedrooms to nest neatly into the roof of a Californian bungalow. Of course this is the bottom end, and it could go easily go a lot higher depending on the structural integrity of the existing building, and how much of downstairs needs renovating at the same time.

If you engage a design professional to complete a proper measure up and concept drawings the builders price will be more accurate, but still not very accurate. Even if you engage a design professional for full working drawings and an engineer for the structural there will still be a little bit of variation as there are still some unknowns.

Converting an attic space into two bedrooms (and possibly a bathroom?) is not a straightforward process - it is actually quite a complicated 3D puzzle, and lots can go wrong if it is not measured accurately, designed well and drawn up to a good level of detail. Structural issues, head height issues, stair design, roof pitches, window flashings, insulation, overlooking, street elevation etc are all important issues that need to be carefully considered. For many draftspeople, this is beyond their normal type of work, which is why I would recommend that you use an architect or building design professional that is experienced in attic conversions.

In your situation, I would engage a local design professional, preferably one who is used to more complex extensions, to design it to concept stage, then get your builder to fine-tune his figures. Ideally a building professional who can model it in 3D can be a good insurance policy that it all fits together as it should. If it all looks good then proceed to a more resolved design, and then full documentation.

Ocean Avenue · More Info

Converting attic space is always going to be an expensive way to create more space, so it is crucial that you spend money at the start to get the design right. It is even more expensive if you try and skimp on design fees by engaging an inexperienced draftsperson, and problems with the design are only realized during construction. Unfortunately, I have seen this happen too often ( I often get called in when projects go off the rails by others) with head height clearance and stair design in many attic conversions, and it is very expensive to rectify.

Best of luck with your budgeting,

Dr Retro of Dr Retro House Calls/Dr Retro Virtual Visits

a good design professional may also be able to suggest alternative options as building in the roof space is expensive as you first have to demolish what's there and rebuilt the roof and the new rooms but adding upstairs also loses downstairs space for stairwell and access..is there any space to extend or repurpose other rooms and/or a garage?