Build on top of garage with timber frame?

Question

HiWe're in the process of house hunting and have come across a niceplace with room for improvement.The property is about 20 years old and has an attached double garage.To make more upstairs space I would like to build on top of thegarage.The owner reckons the existing garage foundations are good enough asthey are, but the garage is just double layer brickwork (no blockwork/cavity).In the upper storey access would be obtained by knocking through abathroom to make a short corridor into the new rooms.I guess there are two ways to do it:1) Increase foundation strength and put in blockwork.2) Use an internal timber frame to support the additional weight ofthe upper storey.Assuming planning permission was granted (a risk to assume I know)then I wondered on the approximate cost for this?  Would anyone haveany idea how much to get the structural work done to a bare plasterlevel?ThanksMarc

Mark · Accepted Answer

I'm in the process of doing exactly this. Going through planning at the moment. Before I got plans drawn for planning, I dug 3 trial holes to expose the footings and had Building Control have a look. They seemed mostly interested in projection, ie., how far the footing comes out from the wall. 150mm is good.

If the footings are not good then some sort of steel framework is needed or underpinning. From what I could gather it would be cheaper to knock the lot down and start from scratch than to underpin.

Your garage walls are unlikely to be the same overall thickness as what will be required by Blg. Control for the new build as you'll need a cavity of say

75mm. That means you've got another little problem to resolve. I suggest a couple of trial holes are the first step.

mark

marc_ely · Answer

HiThanks for the helpful response.So assuming you get your planning passed, what sort of price are youlooking at to complete the job?Are you building a light block internal wall to meet the bldg reg air-gap requirements?ThanksMarc

Mark · Answer

I haven't worked out the total cost. I will project manage my job so should be cheaper than getting a builder.

The cost will consist of three elements:

Plans, Planning, Building regs. Say £1000

Labour
Materials

I am in the process of putting it all into a speadsheet. Guesses at first then refining as I get better information. Example:

Blockwork at say £7 per sq.m plus £12 sq.m to lay. Windows supplied and fitted £1000 Scaffold £1000 Timber for roof £1000, carpenter £1000 Render external walls £1000 Roof tiles, battens, felt. £25/m plus labour.

Also cost in:- Cement, nails, screws, sand, additives, wall ties etc. Lintels, doorframes joists, floorboards, doors, plasterboard for ceilings. Plumbing, electrics, skirting, architrave, internal rendering and plastering, paint, window sills.

You'll soon get up to £20k

mark

marc_ely · Answer

Thanks for the information - the costs are about what I expected.All the best.Marc

John Rumm · Answer

If you built the top bit with studwork then you can meet the thermal performance requirements easily enough in the thickness of a couple of courses.

John Rumm · Answer

You can apply for outline PP on a place even if you don't own it. So you could do that before committing to buy.

Build on top of garage with timber frame?

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