Building workshop & office in back garden - Planning?

Probably not, but you're expressing the dimensions wrong.

What matters is: roof type (pitched or flat), highest point of roof relative to ground, internal floor area.

Internal floor area must be <25sqm taking into account all such structures (although if you also had a cheap shed and wanted to become compliant you'd just knock that).

Height must be <2.5m (flat) or <4m (pitched).

edit:

The external measurement requirement is a bone of contention, as it is internal for a house extension.

Indeed!

Isn't there also a requirement for X amount of garden space to remain? Or that's what we were told when buying our property. The shed size was under the threshold for planning, but because the remaining garden space was less than X it needed planning.

There is also a requirement to have a certain amount of garden space left after its built. either way the floor space of your proposed build is far too big without planning.

Was the advisor selling sheds for a living?

You can do anything you want once the neighbours don't complain. My neighbours went to war over a shed, well they were at war before hand. That's were my advice about the 25sqm limit being the external measurement comes from. Actuall hard limits imposed by the County Council inspector. Told to me by the lad who was very narked at having to chop 30% off his newly finish shed.

This is also my reply. Saves me the bother of typing a longer response.

Whether you're rural or urban or in between may be relevant, from neighbourly pov. Workshop may imply noise but if DIY workshop, hardly an issue.

From a practical pov, an issue may arise if/ when you go to sell. In which case, allow for that it might have to be knocked or modified then.

It mustn't be living accommodation and of course, as above planning is legally required.

losing 10sqm between outside and inside. How thick are the walls?

how much garden will

You have left? Some councils require you to have X open space

OP hope you don't mind me jumping on your thread but an odd question sprung to mind over the mention of "Temporary Structure".

Is a polytunnel a Temporary Structure? Does a large polytunnel need PP?

Many smaller polytunnels are temporary as they have no foundations other than metal tubes in the ground but as they get bigger some are far more permanent with bases that are concreted in.

The construction forum might be a better option for advice rather than DIY

Well, you got professional advice on this thread already but looks like you chose to ignore it. Im not even sure why you asked the question in the first place as you have now indicated that you intend to proceed with the proposal.

3 metres high is the max for that type of roof to keep it within the exemptions so you will be fine.

Just jumping on this. I'm building a garden room but Im just looking for clarification on this

5. FRONT LINE OF THE PROPERTY

Garden rooms can be built to the rear or side of your property. If you would like to place your room at the side of your property, to meet the exemption criteria you must either screen the garden room with fencing, bushes or trees, or the garden room must match the façade of the existing building.

The structure is been built in the rear of the garden but is in line with the boundary so can be seen from the front but my side has a 2m gate. Is this ok so?

I'll just show you what I mean, its easier. I presume that is still classed as rear and not side, despite in side view. Pretty common placing for a shed etc I presume?

Thanks a lot. Since you know your stuff, where is the 3m height taken from is that from the highest point of land around the structure? Or are the rules not that detailed.

As what Exhaust said but be aware of an issue which can arise if shed in built close to or on boundary wall that u share with a pi$$arsed neighbour.

I built a shed to the 3m, he removed 500 mm of clay from along the wall on his side, I had to drop it 500 and he refilled.

It was back in the day when you didn't even need to put it in a brown envelope

Yes measure height from undisturbed ground level at the highest point and you can't go wrong.