Front gardens are a little bit more complicated. So get an architect in.  The height of your fences and the street view comes into play. For tight spaces or odd shapes you might want to go for one our our rotunda or domed igloos; the more exterior space we can leave the more usable the garden still will be. Check here; to see what the minimum garden dimension are for each igloo. This one is 4472 x 4472 making a 20m2 square the needed PD zone. 

In this example there are no residential boundaries. No neighbours to annoy - this can be parks, agricultural, etc; if the non-neighbour side is a public footpath, or a public roads; but what is nice about D12 is the shape; which is not just for looks. 

If you already have an extension, or another shed you want to keep; this reduces the amount of space you have left; if you extension is built on half the 'original' plot of the exterior - well done; but you have run out of m2. Most people with previous developments will have some m2 left; we can build on anything bigger than 5m2.  

In this example; the extention was built on a quarter of exterior plot 10m2; so the largest you could build would be another 10m2 space - D10 for instance; a dome with a decagon base, leaving 10m2 of space. 

Imagine an L shape fence that goes up by 2.5m; and then cantilevers in by 2m; and then that edge is a high voltage electric fence. We need to duck under this invisible boundary. That's all the rule is really. In the example of D10 - a 50m2 is needed - and we can built up to 4M in the unshaded area. Here we build to 3.6m. 

It is a myth that "everything must be under 2.5m" - Only the eaves need to be below 2.5m - here our eaves are 2.1m as it is defined as "where the vertical meets the horizontal" - it is also a myth that only certain roof types are allowed; and different roof types have different maximums.

Just don't trip this invisible perimeter; and the LPA will not be able to enforce a removal; and of course if your garden has a field on one side; this side doesn't count. 

In this example there are residential boundaries on all sides;

1. The original plot of exterior space was100m2; 

2. The previous owner built a 40m2 extension - 4/5ths of the PD is already built on.

3. So, 10m2 - D10 fits snug, 

4. We want to maxumise space in the three dimensions, so we want to built up to 4m,

5. But we cant build right next to the fence, any higher than 2.5m So instead of having a 2.5x2.5x3.16 box, we build D12;  and we preserve views; perserve the pathways and preserve the natural look of the garden too.  Or if that is not your cup of tea; something else? 

In the example of r6; all sides can be built againt all boundaries; as nothing is above 2.5m; the permitted development area must be 30m2 - as it takes up 15m2. 

1. The original plot of the garden was 60m2; 

2. The previous owner built a small extension of 15m2

3. So there is 15 m2 left to be built on.

4. We build H6; a 14.9m2 igloo. 

Consider when building an extension if you are going to leave room for a shed or not. And if you want to build to the maximum of 15m2; you can also build a roof higher than 2.5m - its just all about the positioning. Back gardens are easier than front gardens.