What is involved in building underneath a house?

We are looking to buy a post war home in Holland Park which has a rumpus room underneath which isn't legal height. The rumpus area is divided into 3 rooms which run under the entire house. We want to know whether it's possible to dig into the concrete slab to make one of the rooms 2.7m tall. The room we're considering is no more than 50sqaure meters. The outside walls of the rumpus are concrete and the ones dividing the rooms are plaster boards. We have no idea whats involved, the kind of things we need to consider (especially structurally) or even a rough ball park figure of the costs, who we should contact/get approval from...the most renovation we've done at our current house is installing a shelf so this is an entire new ball game. Any advice or pointers in the right direction would be greatly appreciated.
Thanks in advance!
Ola

Or maybe someone could just recommend where we should turn to dig into the concrete? Any pointers would be great

I can't get a picture at the minute, but do you mean.

House on slab
Slab on earth
Rumpus under one side of the slab?

If so, then you are in for trouble. Get a building inspector to check the place out. They will recommend any repairs that may be required and tell you things that may need attention in the next few years.


If the house was designed to have the rumpus under the first floor, then what makes it "not legal height"?

No you cannot dig in to that concrete floor to reduce the thickness and increase the roof height. Most slabs are no more than 300mm thick so you be in to the soil to get an extra foot of roof height.


Cliff notes:
Get a building inspection by a licensed inspector registered in your state.
Talk to said inspector about the issues he raises and maybe also speak to a builder.
If the current owner refuses to allow an inspection. DON'T BUY IT. the only reason anyone would refuse is they are afraid of a problem they know about being found.
Google "Building inspections in Holland Park" Just copy and paste it

My niece bought an old queenslander & rather than raising it up, they dug out underneath to make it legal height & built it in. She said it was considerably cheaper than the cost of raising it, but in her case underneath the house was open (ie not built in). If you were considering removing what is already there & digging it out, you will have to factor in the cost of completely pouring a new slab for the whol area as I doubt you could get away with doing anything less. (if I understand correctly what you want to do)

@MrsDB that's exactly what we had in mind. Its good to hear its been done before, but because we would have to remove the slab first it may be tricky and have some limitations in terms of how deep we can go.

@chrisandkate thanks for the tips. 'legal height' only matter when you want to rent the room -it's not an illegal room, just in terms of renting and also selling your house -you cant advertise these rooms in the 'official room count' of the house. the rumpus room is above of the concrete slab, we were thinking of removing the slab, digging into the ground and putting another slab done a little deeper so the room is highter, but I'm beginning to wonder whether this is all worth the effort. I have a lot of research ahead of me.

cheers!

What height is it and was it done with permits originally? If it has permits I doubt you would be limited renting or selling it.
We built an extension and due to flat roof it was attaching to the most height we could get was (from memory) 2300mm. It was underheight anyway. We were able to get a permit as the draftsman called it a 'store room' on the plans, and this type of room can have a lower ceiling height. There are no restrictions on us renting or selling.
If you do want to lower the floor I would consult a structural engineer and an experienced, knowledgable draftsman who is well versed in the building code in you shire