Neighbours planning massive extension, she thinks it's only small.

Coopsy85

tacpot12 said:

Any extension that doubles the size of the property (which this sounds like it is doing) is a large extension, and she may find that she is liable for a Community Infrastructure Levy due to the size of the extension. 

You can search the Internet to see if your local authority makes Community Infrastructure Levies and on what size of developments. If they do, the additional cost may cause her to reduce the extension to a size that means that there is no levy. If I were her, I would pay the levy if I needed to do so, to get the size of extension that I want.

Your best route is to make your objections via the planning process, and help others to do the same by offering to show them how to register their objections online.

She will be able to get extension relief on her CIL as long as she gets all her paperwork in at the right time. 

dander

Big/small doesn't really matter. "Out of proportion"  can matter in a planning application. Visibility will matter - if everything is round the back then it's more likely to be allowed. If the size at the back is the worry and it has negative effects on your garden space - then you have to work a bit harder to really describe those impacts. Something that adds bedrooms, therefore occupancy which could impact parking can also matter. 

yksi

The label of small/large is immaterial. If nobody in the street has had an extension of anywhere near a similar size, that on its own might be grounds for being denied, particularly if neighbours lodge objections.

When you're buying a home and thinking of extending, you are always advised to look around you, to see if there is established precedent, since this is a good indicator of likely success. Conversely, an extension out of proportion to both the original footprint and the neighbouring properties will make her less likely to get approval, especially if it leaves little outside space when neighbouring properties have quite a bit.