What do builders need to quote for house extension?

BrokenArrows

So im planning a house extension.

2 floor extension made of a basement and a ground floor. 4m x 7m extension.

The main reason for the basement floor instead of a standard 2 floor extension is because its an east facing garden and the extra height would cut out the sun too much. Im willing to pay the extra of a basement construction to avoid that.

So my current understanding is:

1. I should get an architect to draw up my plans.
2. Apply for planning permission.
3. Approach builders to quote for the work.
4. Accept a builder.
5. Get it built.
6. Done.


My main question is:
What do builders want from me in order to quote for the work accurately? Will architects drawings be sufficient? I assume we need to detail what sort of finish he needs to provide. eg. Build to bare plaster and ill do the rest or specify the paint colour he needs to use. I suppose i need to specify the exact location of things like power sockets, external water taps, internal pipes locations for the kitchen waste and water supply. How details do i need to get and when?

and

What should i expect from a builder along with the quote? Do we draw up contracts detailing the exact work to be done. How is payment usually made. Deposit, Payment at the end etc.

Calahonda52

This will be a big budget movie.

Will you get planning fro a basement?
You will need serious structural engineering advice on the basement and underpinning the house foundations.
Soil conditions will need to be assessed, 3 meters down.
You will be shifting maybe 100 Cum of earth: the LA may stipulate that you need to accommodate it on site and not send to landfill

ECO_Mental

Love your enthusiasm and your get it done attitude....but as the previous poster said this will be a big project considering you want to do a basement. A simple extension is one thing but digging out a basement is a whole other thing. you might have to put in sheet piling or something like that to hold up your existing house.

This is going to require serious professional engineering, but if budget isn't an issue then the stages I would do would be

Get an Architect to do up the plans and the concept drawings.
Get a QS to do up a budget cost based on these plans, if this doesn't scare you off then proceed to the next step.
Get Arch to do planning drawings and get planning.
Once planning achieved get your engineering team and Architect to do construction drawings and structural drawings.
Now you can either tender on these drawings or get a QS to do up a detailed BOQ and tender this. This is a big job and will require specialist contractors and contractors like pricing off BOQ so you will get more interest if you do a BOQ.
Get tenders back and review and negotiate a final fee with your contractor
Get job done


Good Luck;)

Calahonda52

I agree with ECO, I thin k you need some feel for the engineering costs before you go too far, what if you meet granite 1m down... or a council water main/sewer etc
I have been around this space for c 50 years and the one constant is the surprise that ground works throw up.
Let me draw an analogy.

when I had biopsy looking for Prostate cancer, they took 12 samples like on a dart board.
The pre procedure advice was if we don't find it, it doesn't mean you don't have it.
[ You can read about that journey in my profile]

BrokenArrows

Thanks for the information.

Yes I'm aware it's a big job, but not as big as you make out. The basement will not be under the existing house, just under the extension. So there will be no need to underpin the existing foundations.

Yes clearly searches for the land will need to be carried out to see that there is no sewage, water or gas mains running through it.

Obviously if we find any of that or slabs of granite then we will re-evaluate.

Mango Joe

BrokenArrows wrote: »

Thanks for the information.

Yes I'm aware it's a big job, but not as big as you make out. The basement will not be under the existing house, just under the extension. So there will be no need to underpin the existing foundations.

Yes clearly searches for the land will need to be carried out to see that there is no sewage, water or gas mains running through it.

Obviously if we find any of that or slabs of granite then we will re-evaluate.

I know next to nothing about building houses..... but I really do think that people are concerned that if you pay someone to excavate a large cavern beside your house then your existing foundations, now lacking the lateral support they've previously had since the dawn of time, might subside, tumble into this novel void and eventually cause your house to collapse.

To be frank, it's a little unsettling that you're swinging between asking innocent questions and telling people they're "making things out" to be a bigger deal than they are..... The structural integrity of your house and safety of your family is surely a very, very big deal.

Lumen

A small 2-story extension above ground can easily cost over 100k.

I'd make a wild guess that this would cost 200k.

For it to make sense you have to value light in the garden over light in the house, since the basement will be...dark.

So you end up spending 200k for a well-lit space that would cost 50k as a single storey extension, so your Fritz suite costs 150k.

This is why nobody adds basements unless they hate money or live on the most expensive land in the world.

Would this not be better in the Construction/Planning forum?

Lumen

https://www.independent.ie/life/home-garden/interiors/super-basements-the-art-of-digging-down-34574517.html

In terms of cost, building underground is very expensive to do, so it only makes sense if things are really tight in terms of land mass. Cost-wise, a good rule of thumb is that anything underground costs almost double the equivalent above ground. Generally we say above-ground costs are in the region of €1,800-2,100 per sqm, and they would average €3,000-3,500 per sqm below ground.

2016 costs, so whack a bit on.

RandRuns

BrokenArrows wrote: »

Thanks for the information.

Yes I'm aware it's a big job, but not as big as you make out. The basement will not be under the existing house, just under the extension. So there will be no need to underpin the existing foundations.

Yes clearly searches for the land will need to be carried out to see that there is no sewage, water or gas mains running through it.

Obviously if we find any of that or slabs of granite then we will re-evaluate.

The excavation of a basement extension will require extensive supports to the existing house during construction, as well underpinning to support the existing house afterwards. Even then, you'd be taking a chance on what is under the existing house, and would require a lot of (very expensive) ground investigations - if you could find someone willing to take it on. Adding a basement extension to an existing house in Ireland would only make sense if the ground conditions made it desireable (i.e the house is built into the side of a slope or something). The extension you propose would almost certainly be more expensive than knocking your existing house and rebuilding it bigger without a basement. I would go back and rethink this if I were you.