Cost of getting a 2 storey extension on side of house

[Jennywren8]

We have four children and the two youngest share a bedroom but are getting sick of being together and arguing a lot more. It's unlikely my eldest will move out of the house for at least a couple more years (if ever!)). We are a detached house with a garage on the side. We have enough space to knock down the garage (which isn't load bearing) and build a two floor extension on the side of our house but literally have no idea how much this would cost. We live in the midlands. Would anyone even be able to give me a ball park figure so I can work out if it's in the realm of possibility. Thanks!

Terrible picture but hopefully shows what I am talking about...

You are probably looking at around 200k at least.
If you estimate around 2.5k per sqm it would give you a very rough idea.
New kitchen and bathroom costs would be on top of that....a kitchen could cost you anywhere between 15 and 50k depending on size/finish etc.

I did. 2 story side extension in 2020 creating about an extra 75 sq m downstairs and upstairs. I also did 2 bathroom refits within this. It cost 110k

Bear in mind that costs have rocketed since Covid, which most likely accounts for the difference in the two figures above

Could you not just convert the garage? Tons cheaper and it could be a cool big bedroom/studio space for your eldest (especially useful if he's staying long-term, include an en-suite), then one of your two youngest gets the eldest's bedroom. Otherwise the proper extension two-storey thing will very likely end up costing you more than buying an extra property.

This is an excellent idea @pinkdelight, we've just checked the garage and apart from having to move the boiler it could work. We could move our current garage forward as we just have a pointless space in front of it and add a door from the living room into the new bedroom. Thanks for the inspiration, no idea how much it would cost though!

You'd need to check if a garage conversion would come under permitted development or if you'd need planning permission. Building Regs may require extra insulation etc.

https://www.checkatrade.com/blog/cost-guides/garage-conversion-cost/

Garage conversion would be alot cheaper at 10 -20 k

even a garage conversion is expensive now. Ours is costing 20k plus and it’s tiny and already has plumbing and electrics

20k is a helluva lot cheaper than a two-storey extension and adds great value with it's potential for granny annexe/separate space for visitors, older kids etc. Moving a boiler isn't that bad either. We had ours moved from the kitchen to upstairs no problem and not that costly in the grand scheme. Good luck with it!

agreed but our 20k is for a tiny 3m x 4m room with electrics in place, no need to insulate the slab and no need to change the doors into a window (it’s currently a workshop). That doesn’t include any finishes, Internal doors, architrave. Skirting, lights etc all of which will add up..

This is really helpful inspiration. The garage already has electrics and would think just a window at the back and not a door plus some roof windows for some light. Here is what I am talking about if anyone else has some good ideas...

How old is the house? Is your floor likely to be insulated and have a full dpc?

Guess this would need to be insulated

1980s floor is concrete

Not sure what a dpc is?

Damp proof course.

You will have to insulate that floor and put in a dpc. The floor might need digging out to do that. The walls and ceiling will all need to be fully insulated (insulated plasterboard is the easiest way).

You might want to add an insulation and damp proof layer to the floor and floor over. The concrete will be really cold without something. Looks like a lot of space above that could be used for storage for your DCs things. You will need to insulate the walls and roof, to make it comfortable.

You could get a couple of quotes for the full extension and for the garage conversion and see how it stacks up. The full extension will give a lot more space and add more value.

We've just converted our double garage to a kitchen. It cost about 30k . Ours is also 1980s house. We had to get planning permission but we were lucky as the foundations were good (lots of garages don't have sufficient). The roof will have to be replaced and another layer of bricks. I do agree that this does make sense rather than knocking it down. If you have the foundations you could just build on top though
We did a double storey at our last house in 2017 and it was around £130k. Prices have skyrocketed.

As a matter of interest, how do structural engineers/surveyors establish whether or not existing foundations are sufficient to take extra load?

Re moving current garage forward.
If this means a new garage forward of your property line ( even if it’s still on your land ) the planners may not give permission.

Look down your road. If a new garage would protrude past the building line more than other properties in your street / area, the planners may not give approval.

They will request a hole to be dug to check the existing foundations.
It is always assumed a garage has not been constructed to take an extra storey…..why would it.

As an aside.
Garages are generally constructed at a lower level than the ground fl level of your property. ( this is because garages are regarded as a fire hazard, oil petrol leeks etc )
As such you might find there’s room for a dpm and insulation without the need to dig out.
Alternatively, there’s nothing wrong with having a step up into the converted space. Digging out is expensive.

Re roof insulation.
In order to achieve the required insulation to meet building regs.
If your floor to ceiling height isn’t enough you can have a warm roof. which means the insulation goes in above the existing roof. The usual method ( and one all builders like as it’s easier for them ) is to put insulation below the existing roof but that will reduce your fl / ceiling ht. There’s no difference in cost.