Fitting out a garage extension as a utility room, is this feasible?

[Jungle Jim]

I moved into a new place recently, 80's property, where the garage has been extended, firstly back out into the garden, and then subsequently sideways around the back of the house. The second extension has the garage pedestrian door and the original house back door opening into it. A new door was fitted opening into the back garden. This has formed a room about 4m x 2.5m. It has been left internally as very much a garage extension, so bare concrete floor, flat roof continuation from the garage with open joists, single skin walls (one to the garage, two to the outside). The outside walls are rendered externally from about 500mm upwards and painted with masonry paint, inside is 90% rendered (not plastered).

The floor is one brick lower than the dpc but slopes up away from the house. The external door frame sits on the dpc (at the'deep end'), so if I wanted to put a floor in I've only got about 6.5 inches of space before I foul the opening door. To keep a level that decreases to about 3 inches at the 'shallow' end.

I would like to convert it into a utility space for the washing machine, dryer and a freezer. Not a full blown utility room but a better looking space that feels more part of the house than the garage. I would therefore like to floor it, make the walls look better and fit a ceiling.

The only way I can see of flooring it would be to put 3x2 treated timbers as joists, spaced 12 inches apart, with frequent stringers and support the lot on wooden pillars cut to compensate for the changing level every 12 inches or so. Then top that off with chipboard and either lino or wood veneer flooring. As there are no ventilation bricks I thought that to allow air to circulate underfloor I would leave a continuous gap of around 3 inches around the outside of the floor and cover that with a strip of perforated stainless mesh. I intend to fit a humidity-controlled extraction fan to keep condensation down to a minimum but not planning on any heating.

So my questions are:

Is this feasible or even sensible?
Are the wood pillars just asking for rot or is there a better alternative?
Should I still insulate under the chipboard even though I will have a gap around the periphery?
Is there a better method for the whole scheme?
Would I need building regs or planning approval for something like this?

Any advice will be gratefully received.

 

[endecotp]

You probably shouldn't let your washing machine freeze. Humorously, freezers also don't like freezing temperatures. So some form of heating, even if just an electric "frost" heater, is required.

Underfloor ventilation needs to be to the outside air, not inside the room.

An alternative to a ventilated suspended floor is a solid floor; lay a damp proof membrane over the existing concrete and up the wall to meet the existing DPC. Then fill it in. One option would be enough sand to level it, then insulation boards, then a floating wooden floor (e.g. T&G 22mm chipboard).

 

[Jungle Jim]

I hadn't considered a solid floor option. Looking at it, a levelled solid base would make my life easier in that it would leave more room to lay insulation and possibly even some underfloor electric heating with a frost stat (I know freezers don't like the cold but I forgot about the water in the washing machine, doh!). I will give that some serious thought, thanks for the advice.

 

[^woody^]

This would be a standard garage conversion with a timber floor. Building regs with cover the detailing.

This would be permitted development unless PD rights have been removed.