Potential extension - budget estimate

[Ecosseven]

Afternoon all,

We are considering building a small extension on the back of our house as an alternative to moving. We have looked at sq.m estimates on line and these seem to vary significantly and was hoping that someone on here might be able to offer some guidance. Some details below.

-House location - East Central Scotland
-House age - 25 years
-House construction - inner timber frame with harled / rough cast block walls externally.

Proposed extension details

-Single room approx. 3m x 4m at ground floor level to be used as extra living space
-Construction to be similar to the house. We want to use the room all year round so not sure a conservatory is the best option for us.
-Flat roof or shallow mono-pitch
-The current house floor is elevated by approx. 400 - 450mm from the external ground level (3 steps at both the back and front of the house).
-Extension will include hard floors to match the dining room.
-Heating will be via wall mounted radiator and tied into the existing gas powered central heating system.

Prices quoted on-line seem to range from £2000 / sq.m up to £4500 / sq.m.

Any thoughts gratefully received.

[DickyC]

After an abortive attempt with an architect, a reputable builder and an estimate of £160,000 for a 6m x 5m single storey, flat roofed extension with roof lanterns, we went with a 5 x 5 orangery with one roof lantern, built by a conservatory specialist for £60,000.

It's a proper build with one metre deep foundations and bricks to match existing. And lots of glass. Very pleased with it.

£2,400 per sq metre.

[Ecosseven]

DickyC said:

After an abortive attempt with an architect, a reputable builder and an estimate of £160,000 for a 6m x 5m single storey, flat roofed extension with roof lanterns, we went with a 5 x 5 orangery with one roof lantern, built by a conservatory specialist for £60,000.

It's a proper build with one metre deep foundations and bricks to match existing. And lots of glass. Very pleased with it.

£2,400 per sq metre.

That's really useful. Many thanks.

[DonkeyApple]

Ecosseven said:

Afternoon all,

We are considering building a small extension on the back of our house as an alternative to moving. We have looked at sq.m estimates on line and these seem to vary significantly and was hoping that someone on here might be able to offer some guidance. Some details below.

-House location - East Central Scotland
-House age - 25 years
-House construction - inner timber frame with harled / rough cast block walls externally.

Proposed extension details

-Single room approx. 3m x 4m at ground floor level to be used as extra living space
-Construction to be similar to the house. We want to use the room all year round so not sure a conservatory is the best option for us.
-Flat roof or shallow mono-pitch
-The current house floor is elevated by approx. 400 - 450mm from the external ground level (3 steps at both the back and front of the house).
-Extension will include hard floors to match the dining room.
-Heating will be via wall mounted radiator and tied into the existing gas powered central heating system.

Prices quoted on-line seem to range from £2000 / sq.m up to £4500 / sq.m.

Any thoughts gratefully received.

Yup. It's not a cheap act any more. Gone are the days of knocking up a single skin box over a weekend and some shed roofing on the top.

The cost range you've got there is about right as a basic box to regs will costs around £2k. But projects vary and some involve internal walls being removed, the extension being more than one room, having more than one or two windows or the space having a bathroom of kitchen installed etc so then you start going north from that £2k quite quickly.

Something else to consider these days is that as the base cost is so high due to the floors, walls and ceilings all have to be to modern regs, one does need to consider the footprint quite carefully as firstly, below a certain size it really starts to make sense to be considering 'orangeries' as an alternative and at the other end of it only costs a little bit more to add more space, even if you don't need it currently, then it's worth considering. It's all too often smarter to spend a bit more while already paying for all the labour and equipment to go a bit larger than to try and save a few £ by going smaller.

A basic box with a couple of windows or a window and a door is probably somewhere in the region of £2-£2.5. And you can always do some of the fit out work yourself although the final cost to slop some paint and put down a floor will be negligible by contrast.

[NDNDNDND]

This is quite an interesting tool for rule-of-thumb type cost assessments:

https://costmodelling.com/building-costs

Bear in mind you do have to go through the whole calculation process to get a reasonable estimate.

£3k/m² + VAT is a reasonable baseline at the moment. Therefore yours would be approaching £50k + VAT.