Garage Conversion

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Hi All,


Looking for some advice to better educate and equip myself before getting some builders in for quotes for a small part of our conversion.


Could you clever people read through and advise please if anything doesn't sound right or could be improved.


We are planning to convert our single storey attached garage to a bedroom and also extend upwards.

The inner wall is against the house, so no issues, the outer wall is mainly single brick but has double brick "pillars" every so often.

Our architect has suggested that the inner wall can be built on top of the floor slab level against the "pillars so it will create a cavity wall in the majority of the wall.

We then will insulate the cavity and also consider a small amount of insulated plasterboard on the inside. The door end will be bricked up with a window.


Once the wall is built we will need to raise the floor about 120mm to become level with the rest of the house. - the garage has an open gully trap drain in the corner 

that connects direct to the sewer and has a waste pipe from an adjacent WC sink flowing into it, this will need to be sealed up.

We have planned to put a wet room en suite in the conversion - the toilet and sink waste will be routed through the end wall so that is not an issue.

The shower waste will need to be much lower so my thoughts are that we could connect this to the gully drain as we do not seem to have any other options unless

we raise the shower height.


How would you guys raise the floor, concrete, wooden frame etc? 

If using concrete (my preferred option as it would be solid) would you sandwich a thin layer of insulation board? say 20mm between the existing slab and new floor?

The shower waste will be diagonally across from the drain, so when digging out the floor would you connect diagonally across which would be the shortest route?

Would you use 50mm solvent weld waste pipe?


Hope this all makes sense.





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4 hours ago, wozza said:

..........and also consider a small amount of insulated plasterboard on the inside.

Building control will tell you what goes where, it's not for 'consideration' ;)

If your building a block internal then you'll be tying the leaves together so get an angry man with a BFO petrol cutter to cut back the buttresses ( pillars ) and fly right through with tied and filled cavity walls. 


Next is the floor. 

I very much doubt you'll be allowed to full fill with concrete as that won't pass BC either through lack of insulation. 

On the last one I did, I laid 125mm iirc of EPS directly onto the garage floor over the DPM ( checking it was level first which it was ) and then finished the floor with 2 layers of 18mm P5 floorboards. That felt better than walking on a timber joisted floor, very solid.

The internal cavity walls were covered with 65mm insulated plasterboard dot n dabbed over blue grit. You could prob get away with thinner boards on the walls if you cut back the pillars and full fill the cavities. 

Sounds to me like you need a chat with your BCO first, then come back here, and then get a builder out once you've decided on the method of construction. 👍


Edited by Nickfromwales
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Thanks Nick.


We have got a few builders coming over the next week or so to have a look and give rough quotes etc, will be interesting to compare what they all suggest.


Edited by wozza

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Let them do the talking and write everything down, per builder. Post the outcomes here and we'll chuck our 2-Penneth in 👍

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