Recommendations for replacement and construction of garage with workshop / garden office

[Dojo]

Thanks for welcoming me to this forum.

 

I'll start at the beginning.

 

I have an old brick-built garage resting on a concrete slab. The slab is cracked in several place creating some subsidence which in turn has cracked the garage brickwork etc. It needs replacing and provides an excuse for us to knock it down and extend the footprint to encompass a small workshop with office / chill room (partner would like a toilet in there with sink).

 

Being new to this, I’m thinking that demolition and removal of the slab and brickwork (plus asbestos roof) and replacement with a new concrete floor. The reasons for this is if I sell the house I would like to offer the purchases the option to use the building as a garage if they so wish (which I believe rules out a floating / suspended floor). I do not believe the foundations need to be particularly deep as I am considering a wooden construction on top and that if it comes to it, the weight of a vehicle.

 

I was quoted sometime back a brick skin on a simple concrete base for around £20k without any extras. For that, I feel I can build something better and have since considered a timber frame construction.

 

I have been looking at garden offices etc. and feel that many of them, especially the cheaper variants appear rather lame in that they have poor insulation and do not appear particularly sturdy. It needs to last, be warm / cool and comfortable (as well as look good). This is what I am considering thus far and would welcome feedback / recommendations as to the material etc., used and the construction procedure. I have put this list together through various searches. I recognise that some of the questions relate to price and so if you could mention efficiency alongside equivalent (cheaper) then that would be fantastic.

 

Foundation

Type 2 hardcore

DPM

Concrete

 

Wall

45 x 75mm joists in the floor.

70 x 70mm C16 timber posts with 70mm insulation (directly onto the concrete) OR 45 x 95mm C16 with glass mineral wool / rockwool.

Plain plasterboard.

 

Roof

Soffit - 9mm ply.

Roof sheathing - 12mm OBS

insulation - 150mm wool (or double line with 70mm to save costs as already ordered for walls?).

45 x 195 mm rafters.

Foil backed plasterboard.

 

Windows

Double or triple glazed UPVC.

 

I have some additional questions, if you are able to make recommendations then I can have a look around and make comparisons:

 

Can you recommend a decent roof sheeting that resembles tiles?

SIP, traditional structure (as above)? 

Larch or cedar external cladding?

How will counter straps be used in the wall construction?

Read about reflective building paper going into the wall construction. Do I need this and if so, where would i place it?

One construction company mentioned the use 25mm ridged insulation in the walls but unsure where that goes?

I haven't figured out the best / more efficient / cheaper way of attaching the frame to the concrete yet.

 

I know I am asking for a lot, but I believe in providing detail when asking a question. I do hope you won't mind me asking. 

Edited November 13, 2022 by Dojo

[Conor]

Before you go down this route, check to see if this falls under permitted development rules and what building control regs and process will apply, and these will dictate you construction spec. Eg. Its to be used /can be used as a habitable space (you've mentioned bathroom etc, so almost certain BC will apply), then you'll need to meet certain building regs, e.g. insulation, prevention / spread of fire etc. Do this first before picking a construction method.

[Dojo]

Thanks for the feedback. I've already contacted them and they've been extremely helpful. I would to estimate costs alongside this.