Let's start at the beginning then:
JimJam":2nleqpxf said:
Hello. I'm going to assume your workshop is your 7m x 9m ideal - in most cases it is not critical, but it is certain that it will exceed the de minimis 10m3 allowance which applies in all circumstances - so you need to ensure that you satisfy all the other exemptions.
I'm also going to assume that you are not in a Conservation Area, Area of Outstanding Natural Beauty or a National Park and that your house is not a Listed Building? If you are in any of those things, you need planning permission for anything bigger than 10m3 so the rest of this post is redundant.
I'm then going to assume this is for domestic use - if it is for a business, you need planning permission.
JimJam":2nleqpxf said:
I'm planning to build a new workshop in the next few months. The workshop will be at the back of my garden which borders a rail line.
Railway line is good, as it means there cannot be a 'highway' along the back, so the workshop should be further from the highway than your existing house.
The only difficulty might be with footpaths, bridleways etc. Are there any of those in the land adjacent to your garden?
If not, then you meet the 'no part of the workshop shall be closer to a highway than the existing house' exemption.
JimJam":2nleqpxf said:
The workshop will be 35meters from my house
That's good - it will not automatically be classed as an extension then, as it is more than 5m from your house.
In addition, if the workshop is 35 metres from your house then it is probably safe to assume that even after building a 7m x 9m workshop on it, less than 50% of your curtilage (front and back gardens, driveways etc, i.e. all land around your house) will be covered with building.
JimJam":2nleqpxf said:
and at least 50+ meters from any neighbouring property so no problem there.
In fact, that's irrelevant.
JimJam":2nleqpxf said:
The width of my garden at the back is 7.6 meters. If however I have to allow for a 1 meter clearance between workshop and sides/back of garden then my max width will be 5.6 meters.
That is irrelevant from a planning perspective - I'll deal with building control below.
JimJam":2nleqpxf said:
Length is not an issue as I could make the workshop anything up to around 15meters long. I'd like to try and achieve a squarer shape for the shop as I think this will offer a more flexible layout. I think the ideal would be 7 x 9. But this would depend on how close I could go to side/back boundaries.
Proximity to boundary is irrelevant from a planning perspective - I'll deal with building control below.
The only thing that you need to think about for planning permission - assuming my assumptions above are all correct is to design a workshop which satisfies the requirement that the workshop is
not more than 3 metres high, or not more than 4 metres high if it has a ridged roof. (Measure from the highest ground next to it.)
The shape of your workshop will affect that, if you want a ridged roof. If it going to a be flat roof, then you are OK whatever. If you want a ridged roof, then you need to make sure that you can get your roof ridge in below 4m - whether by using materials that allow a very shallow slope (metal) or by reducing the width of the workshop.
However, if you want a wide workshop with a ridged roof in materials that mean the ridge will end up more than 4m high - off you go to planning.
OK, so that's planning out of the way.
Building control - you smash the 30m2 floor area with a 7m x 9m (63m2) workshop so you will need building regulations approval of the workshop whatever you do (unless you reduce the size by over half). The rule about exemption from building regs and 1m from the boundary is irrelevant.
However, Building Control will probably insist that either you build more than a metre from the boundary or that you build substantially in non-combustible materials. Give them a call and discuss that with them.