Extension with rooflights

[jesteh]

Hi,
We are looking to convert conservatory at the back of a 1930s house to an extension. The size of the additional space is 7x3 meters and its been built at the back of the house opening onto the rest of the house with 2 openings size 2.5m by 2.4m (no building regs approval - previous window and back door).

As the extension will make the back rooms of the house quite dark, we were hoping to put in 2 large roof lights (2mx2m) at roof levels as well as a large glazed door (2.75mx2m) and window (2.75mx1m).

Are we going to be ok with doing this and is it going to make the extension quite cold with such large roof windows. Doing the calcs for heat loss works out that allowed openings are 3*7/4+(2.5*2.4)*2=17.25 so it kind of looks ok.

I also heard that sometimes they let you work the heat loss through entire house. Based ont his, is it worth holding off doing the cavity wall insulation before doing heat calcs or do they accept if we just had it done as new (its booked for next week).

 

[freddiemercurystwin]

Your post is a little confusing tbh. The doors have been removed between the house and the conservatory: correct?

How old is the conservatory? What is its construction? Photo? Frankly it will probably more practical to demolish and rebuild it.

What is the 17.25 figure? Is that the glazed percentage area of the floor area?

One assumes you intend getting Building Regs approval for this?

 

[Static]

How old is the conservatory? What is its construction? Photo? Frankly it will probably more practical to demolish and rebuild it.

More than likely a proper exension will be a complete rebuild.. and may even require improved foundations..

 

[jesteh]

25 years ago someone removed the original door and window at the back of the house and added conservatory opened to dining room and kitchen at the back of the house. They didn't contact building regs and its not compliant (and wasn't at the time). We are planning to re-use foundations of this (concrete) and build it up as an extension + make it building regs compliant. Building control have accepted to re-use concrete foundations but we have yet to discuss overall design. 17.25 is heat loss calculation. Apparently, new window and door openings should be equal to quarter of floor space + previous window openings. Alternatively, if you improve existing insulation across the whole house they are sometimes ok with that.

 

[freddiemercurystwin]

Wait for calcs to be done first.

 

[jesteh]

Yeah, but would be nice to know where we stand before we go and speak to building inspector which is why I am asking here.

 

[RonnyRaygun]

Not quite following your calculation.

You are allowed openings equivalent to of 25% of the floor area to comply with building regs. You are allowed to add the area of any existing openings that are now in the extension to your allowable area.

So...your floor area is 7x3 = 21m^2
21x0.25 = 5.25m^2

Plus existing openings = 2 x 2.5m x 2.4m = 12m^2

Total allowable openings in extension = 5.25 + 12 = 17.25m^2 (Aha...that's where the 17.25 comes from! )

Your new openings will be 2 x 2m x 2m = 8m^2 (for the rooflights)
Plus 2.75m x 2.0m = 5.5m^2 (door)
Plus 2.75m x 1.0m = 2.75m^2 (window)

Total: 8 + 5.5 + 2.75 = 16.25 < 17.25 so I agree, seems to be OK.

If not, you can get a whole house calc done, or you can take a worst case notional extension that complies with current building regs (of the same size as yours), and prove that with better insulation in the walls, roof, and floor, your extension loses less heat than the notional one. Can also utilise triple glazing or xenon filled double glazing in this instance.