How do I work out the cubic volume of my house?

Question

I'm drawing up plans for an extension and the local authority want to know the current cubic volume of my house now and also the cubic capacity of the extension.

I know for a cube its length x bredth x height but do you need to make different calculations for the roof / attic and if so, how do you work this out?

Huge thanks, everyone!

Lisa

Answer 1

Local Authorities usually issue guidelines on this. (Check the website). Some may ignore pitched roofs (as they are not living space) others discount basements.

It is in your interests usually to measure the whole volume though as the % you are allowed to extend will be greater. In the abscence of any instruction to the contrary,  this is what you should do. Most planners don't know the rules either so unless it is critical they usually accept what you are telling them unless it is obviously incorrect. You have to measure the extension in same way, of course, which may or may not be an advantage.

Area of a gable is simply half the base of the end triangle times the height times the length of the ridge; this gives you the volume that you add to the rest. (If you know about geometry you will know the pitch is irrelevant, btw). You will need to make an adjustment if the roof is hipped. Some authorities calculate the volume as a flat top box with the height halfway up the roof slope - which is usually about right.

Calculations are gross, that is to say measured externally in each case. You need to be reasonably accurate but not precise to the nearest millimetre.

Answer 2

Well, you have the way for the bits under the roof.  Just measure up the OUTSIDE and if the house is one rectangular box that bit is easy.  If it is a complex shape, divide it into digestible chunks.

For the roof space imagine looking at the end of the house where you can see the angles of the roof.  What you need is to get the volume of the attic which is a triangle the length of the house.  The area of the triangle is 1/2 base x height, and the volume is the area x the length.  

I emphasise the outside measurements because they have limits on the size of extension you can build without planning permission - this is called permitted development and you can under some circumstances extend your house by 10% of its original volume.  Now if it is listed or in a conseration area it does not apply, but permitted development can be a big help if by any chance your neighbours would object to the plans.  If you are within the 10% then you may be OK anyhow.  If you are just outside it, you can tinker and knock down some small part - a garage or porch so that the net change is less than 10%.  So if you are measuring do not underestimate it. If you have someone drawing up your plans they shoudl be able to tell you if you are not sure about this.

Answer 3

the roof and attic space does not come into it ..unless you all ready have an attic conversion...your gonna get a lot of answers about the roof volume being half the width by the height etc ..that only works if the roof is at a pitch of 45degrees ...most roofs are not ..I'm a builder and have put very few on at that pitch

Answer 4

for a standard gable roof you should be able to take height times width times length and divide it in half. A gable roof is basically 2 triangles when arranged properly form a square.