Extension plan - opinions and cost estimates?

Rosa_Damascena

After seeing a post earlier this week about an extension plan I'm going to be brave and put myself out there with my own. My current rear conservator is going to be brought out further and my garage will be knocked down and converted into a kitchen / utility. Both will be built with foundations suitable for a double storey structure. This is an image of a screenshot so I appreciate might not be 100% clear. The footprints of the 2 new extensions will be 8240mm x 4665mm and 4370mm x 7020mm. Its an unconventional plan I know, and there are very specific reasons I want this design.





I'll be supplying the doors, windows, kitchen and bathroom fittings and flooring myself, landscaping (clearing the surrounding area of trees and roots) will also be down to me. The builder will be covering the rest, including all electrics and utilities, relocating the boiler and fuseboard, new floors all through the GF with a wet CH system and a narrow concrete patio area around the new build, roughly 600mm (my choice of paving / finish will be additional). The floor at the rear of the property and garage will need to be raised in order to be accessible, as will an existing door lintel where the proposed downstairs bathroom joins the extension. This bathroom will actually be divided into two - a wet room with toilet and sink on the bedroom side, and a cloakroom with a small lobby accessible from the double front door (which will be at the side of the property). The old front door will be a private entrance for the family and the idea is that guests are contained to the living and kitchen areas, which will be built as orangeries. The whole build and upper storey of my home will be rendered with insulation.


Any idea of how much the build will should be? (Excluding the bits I'll be supplying - I'm budgeting for upto £50k here). I'm based in the South East so expect London prices. 

Doozergirl

Do you have your existing floorplans? I can guess at what is an extension from that, but it's easier with a floor plan, but just looking at it, £50k isn't going to go far.  

You're adding 70 square metres of space, the size of a small 2 bed house.  Even a very conservative estimate of £1.5k per metre on a basic finish of extension takes you to £105k.  

As I attempted to drill home earlier in the week, the cheapest thing to build is a square, so the conservative estimate is unrealistic because of the amount of surface area.  To put it into perspective on the footings, you need 38 linear metres of footings for those 70 square metres of space.  If we count the existing wall that's given for free, that's 45 linear metres of containing walls... 

My new chalet bungalow of 240 square metres has 44 linear metres of containing walls.  

Windows cost a lot of money.  Rooflights cost even more.  Building an "orangerie" costs considerably more than an ordinary extension.  Our average single storey extension these days contains quite a bit of glass and costs around £2.5k per metre without kitchens, ultilities and bathrooms.  

You're adding those, renovating inside, adding UFH, some of it retrofit, which isn't cheap.  

And you're putting external
insulation around the whole house. Also not cheap.  

Have you found a builder for this? 

Did you discuss budget with the architect? 

You were talking about essentially shutting down part of the house and living in another the other day. I'm conscious of asking people why as it can appear rude online, but why?   I feel like like my purpose on this planet is the try and get people to live well whilst using their entire house.  

FreeBear

Doozergirl said: Windows cost a lot of money.  Rooflights cost even more.  Building an "orangerie" costs considerably more than an ordinary extension.  Our average single storey extension these days contains quite a bit of glass

Even high spec triple glazed windows have a low thermal performance compared to a well insulated wall or roof. The more glass you have, the bigger the heating bill.

ComicGeek

FreeBear said:

Doozergirl said: Windows cost a lot of money.  Rooflights cost even more.  Building an "orangerie" costs considerably more than an ordinary extension.  Our average single storey extension these days contains quite a bit of glass

Even high spec triple glazed windows have a low thermal performance compared to a well insulated wall or roof. The more glass you have, the bigger the heating bill.

Yes, even triple glazing would lose at least 3 times the amount of heat than a bldg regs compliant wall/roof. Also have the large solar gains to consider in the summer - nice enough in the autumn/spring, but can be unbearable in heatwaves, particularly with single storeys.

ka7e

ComicGeek said:

Yes, even triple glazing would lose at least 3 times the amount of heat than a bldg regs compliant wall/roof. Also have the large solar gains to consider in the summer - nice enough in the autumn/spring, but can be unbearable in heatwaves, particularly with single storeys.

Solar gain depends very much on orientation. We previously had a W-facing kitchen which got unbearably hot in summer after midday as the sun shone directly in as it passed it's zenith. Now we have a S-facing kitchen and the sun is too high in the sky in summer to penetrate the ground floor. The S-facing first floor bedroom DOES get hot! In winter, the sun is lower on the horizon and we get the benefit of solar gain all day.

The skylights in the plan above will allow the rooms to get very hot, unless they incorporate some sort of blinds for use on very sunny days in summer.

Rosa_Damascena

Doozergirl said:

Do you have your existing floorplans? I can guess at what is an extension from that, but it's easier with a floor plan, but just looking at it, £50k isn't going to go far.  

Original floor plan attached (the open bathroom in B1 is now an enclosed en-suite).

The £50k was a very conservative estimate for the new windows, kitchen bathrooms and floors, going very basic (this is the build element I feel I have the most control over). I knew the build itself would be north of £100k but the question is how close it will be to £200k - or above? That will determine whether the work can proceed next year or whether it will have to wait. Just wanted to be in a position to bat away a silly quote. My preferred builder is VAT registered and warned me that this is not going to be cheap but what I need to decide on is affordability.

I've got a lot of stuff inc home library, and that will need a home during the build because I have decided to go with wet UFH throughout the whole GF. I personally cannot live and work in it at the same time because of the disruption and that's something I cannot compromise on I'm afraid. Not worried about this as I have family that will happily put up with me for a while.

FreeBear

Rosa_Damascena said: I've got a lot of stuff inc home library, and that will need a home during the build

If you retain the garage, you'll have some much valued storage space.

If the garage really has to be demolished, it would make sense to leave it until the extension & interior work is near completion - If nothing else, it provides safe storage for materials & tools.

Rosa_Damascena

FreeBear said:

Rosa_Damascena said: I've got a lot of stuff inc home library, and that will need a home during the build

If you retain the garage, you'll have some much valued storage space.

If the garage really has to be demolished, it would make sense to leave it until the extension & interior work is near completion - If nothing else, it provides safe storage for materials & tools.

The original plan was to do this over 3 years: firstly the conservatory replacement, then the garage, new front door area and bathrooms, and finally the kitchen fitting in the last year. My plans changed because I was told (by my builder) that this would add to the cost by thousands. A quote is being prepared.

The garage is always going to be a headache as not fit for purpose (its falling down and beyond repair) which I knew from the outset. I'm planning on getting a shed this winter instead which should be more than adequate storage space.

So realistically what should the build cost?

Doozergirl

It really depends on your anticipated fittings, but I think you're an awful lot closer to £200k than £100k when it's finished, quite easily over it.  Extensions with lots of glass tend to cost more in the region of £2.5k per metre.  

I wouldn't be compromising on fittings if I could afford an indulgent build route.  In fact despite the relatively square box, we've chosen an expensive build method and I'm not compromising on anything.  😬

Rosa_Damascena

Doozergirl said:

It really depends on your anticipated fittings, but I think you're an awful lot closer to £200k than £100k when it's finished, quite easily over it.  Extensions with lots of glass tend to cost more in the region of £2.5k per metre.  

I wouldn't be compromising on fittings if I could afford an indulgent build route.  In fact despite the relatively square box, we've chosen an expensive build method and I'm not compromising on anything.  😬

This is the kind of straight-talking I needed. I've been working all weekend as I need to build up my funds, and yesterday's 12hr session was particularly tough. I ended the day wondering whether any of it was worth it - would I have any time to enjoy my home v2.0? But since the extra space the extension will offer is ultimately not for me, and its going to involve upheaval whenever it is done you have now restored my vision of a sparkling-new build.

Nose back to the grindstone now .

Rosa_Damascena

Ok, so inching forward here now its 2021.

What's the industry standard guarantee for buildings (extensions)? Is there something equivalent to a NHBC, but for extensions?