This article comes with a health warning – I’m only going to give you general information here, and it’s always worth checking with your local authority if you are at all unsure of whether you need permission. It’s always best to find out first, rather than be forced to apply retrospectively, and potentially face an order to remove your observatory. This article was written in July 2016, so may not reflect any changes in legislation since that time. Finally, it only applies to England and Wales – different rules may apply in Scotland and Northern Ireland. There are some links at the bottom of the article that take you to our sources of information, so you can interpret the law for yourself too.
Observatories in domestic gardens fall under the title of “Outbuildings” when it comes to planning, and like most forms of domestic development, if they fall within certain limits (in terms of size and placement) they will not require planning permission – being considered “permitted development”. Here’s a run-down of the rules that are applied, with some explanation:
Rule 1: No outbuilding on land forward of a wall forming the principal elevation.
This means that no part of your observatory can be in front of any part of the front wall of your house. Any outbuilding in front of your house will require permission. Bear in mind that this refers to the “Original House” – the house as it was first constructed, or as it stood on 1st July 1948 if it was constructed earlier than that.
Rule 2: Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
It’s unlikely that you will build a 2-storey observatory, but it has been known! Plans to build, say, an observatory above a warm room (or on a garage roof etc.) will definitely require permission.
The more important rule here is the height rule – domes do not qualify as “dual pitched” (obviously) so they must be under 3 metres in height, and so must pent-roofed (flat slightly sloping roof) observatories.
So – we can build a 4m high roll-off roof observatory? Not always…. watch out for rule 3!
Rule 3: Maximum height of 2.5 metres in the case of a building, enclosure or container within two metres of a boundary of the curtilage of the dwellinghouse.
A couple of definitions: The dwellinghouse is the house currently on the land. The “boundary of the curtilage” means the boundary of your land – usually the wall, fence or hedge between your garden and your neighbour’s, though your land may be bordered by something else (e.g. a river or even nothing at all) – the boundary still exists.
So – here is the gotcha. If your observatory is going to be within 2m of your boundary, it must be 2.5m high, or less, no matter what type of roof it has. We generally design within a 2.5m limit, so you shouldn’t have a problem there with a roll-off-roof observatory.
Remember if there isn’t a dwellinghouse, your development isn’t in a domestic garden, and the observatory can’t be “incidental to the enjoyment of the dwellinghouse” – so you will definitely need permission (see rule 10). These rules only apply to domestic garden observatories!
Rule 4: No verandas, balconies or raised platforms.
This rule is aimed at preventing you building what is essentially a “viewing platform” for observing your neighbours! Not relevant to observatories of course, as they don’t generally have these features.
Rule 5: No more than half the area of land around the “original house” would be covered by additions or other buildings.
Another definition required here: the “original house” is the dwellinghouse, as it was first built, or as it stood on 1 July 1948 if it was built before that date. So you must add up the areas of land covered by all of your other outbuildings, and any extensions added after 1948, and make sure that once your observatory is built, more than half the land around the “original house” is not built on.
Rule 6: In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from house to be limited to 10 square metres.
This rule limits the size of outbuildings when they are built on National Parks, the Broads, AONBs and World Heritage Sites. The 10 square metre limit applies to buildings that are more than 20m from the closest wall of the dwellinghouse.
Each of these special designations have their subtleties when it comes to applying for planning permission – so it is essential that you do your research if your observatory doesn’t fall clearly within the permitted development rules.
Rule 7: On designated land buildings, enclosures, containers and pools at the side of properties will require planning permission.
Again a definition: “Designated Land” is any of the following:
- National Parks
- The Broads
- Areas of Outstanding Natural Beauty (AONBs)
- Conservation Areas
- World Heritage Sites
If you’re thinking of building at the side of your property (as opposed to at the rear, which is fine) you must get planning permission if you live in any of these types of area. Be particularly careful of AONBs and Conservation Areas – you might not even realise you are in one! Your local authority should be able to tell you, though.
Rule 8: Within the curtilage of listed buildings any outbuilding will require planning permission.
This is a simple one: if you live in a listed building, you MUST obtain planning permission for any outbuilding.
Rule 9: No Living Accommodation or Microwave Antenna
This requires a little explanation – the new building must not be “separate, self-contained living accommodation” – in the words of the planning act, it must be “incidental to the enjoyment of the dwellinghouse” – so you use an outbuilding as another house! That’s not relevant to observatories of course, but it is worth remembering the rule about “microwave antennae” if you are considering how you might connect your observatory to your home wifi network!
Rule 10: The rules above ONLY apply to houses, not flats or maisonettes, or any other type of premises.
If you live in a flat or a maisonette, even one with a garden, then you’ll have to obtain planning permission (and of course your landlord’s permission) to build your observatory. Schools, commercial premises, farm land and every other type of land require planning permission for any outbuilding.
Rule 11: Watch out if you’re on Designated Land!
Even if you meet all of the above requirements, it’s essential that you check carefully for any restricted permitted development rights that may exist if you are on designated land. Local planning authorities, such as your National Park authority should publish this information on their websites.
Rule 12: Look out for Tree Preservation Orders
If you’re building an observatory, tree preservation orders (TPOs) can affect your plans – so you must work carefully:
Any work you intend to do to a tree that is subject to a TPO must be notified to the local authority – this includes cutting down, topping, lopping, uprooting, and wilful damage and destruction. In short, if the tree is getting in your way, don’t touch it without getting consent from your local authority.
You must also take reasonable steps to protect trees that are the subject of TPOs – and that means not digging where the roots might be. Cutting roots is a prohibited activity and requires the authority’s consent.
Some Frequently Asked Questions
Isn’t an observatory a “temporary structure” and thus exempt from planning?
This of course is all down to interpretation! The quick answer is if it is concreted into the ground, it is unlikely your local authority will consider it “temporary”! Sheds are covered by planning law, and most of those are a lot less well-constructed than an observatory, so to be on the safe side, I’d recommend you always assume your observatory will need to comply with planning law.
That said, consider the local authority’s position. If they were to enforce against an observatory that was easily moved or removed, it would be quite possible (and sensible) for the owner to simply relocate it were the local authority to object to its placement. So a plastic dome is evidently a bit more “temporary” than a large timber building.
Where do I measure the “height” from?
The government doesn’t say where exactly this height should be measured from – and different local authorities look at this in different ways. We would recommend measuring to the top-most point of the observatory from the lowest point of the ground around it, to be safe. However we have heard of local authorities making some very strange interpretations, such as measuring from the “lowest point on the boundary”. If in doubt, check.
I am considering a roll-off observatory with a “pergola” type structure that the roof rolls onto. Does that count in the 10 square metre limit?
The 10sqm limit in National Parks, the Broads, AONBs and World Heritage Sites applies to the whole structure. In practice it might be hard for a local authority to argue that it “covers” the ground under the pergola (especially if it is just a simple frame) but we generally assume that they would decide it does – and therefore the whole structure must be under 10sqm to avoid the need for planning permission.
How do I check for Tree Preservation Orders in my area?
Have a look on your local authority’s website (this is your borough or district council, not your county council, but in National Parks, it will be the park authority). Most provide really good searchable databases that will show you the locations of nearby TPOs on a map. If not, just give them a call.
Seriously, a wifi antenna is a microwave antenna?
Technically, yes. Of course you may very well put a wifi box in your observatory. Just don’t mount some dirty great Yagi aerial on the side of it, right? To be honest, I’d usually recommend you run a cabled network out to your observatory anyway… far less frustrating if it’s any distance from the house!
Aren’t there any rules about what an outbuilding may look like?
Seems strange, doesn’t it, but no! If you’re within the rules for permitted development, your outbuilding can be blue and silver, bathed in purple light and surrounded by a moat if you like. Of course that might not make it terribly useful as an observatory. Should it fall outside the rules of permitted development though, you will certainly need to consider its appearance when you apply for planning permission!
Are there any other (non-planning) regulations I should consider?
If it will be connected to mains power, you will need to consider the requirements under the wiring regulations (in particular for the power supply cable and the need for certification/notification). There may be others but observatories are usually straightforward!
Do I need Building Regulations approval?
This is not normally required if you don’t require planning permission. Furthermore, if the observatory doesn’t contain any sleeping accommodation and is less than 15 square metres in size, building regulations will not normally apply. Finally, if the building is more than one metre from your boundary or is constructed of substantially non-combustible materials, AND contains no sleeping accommodation, building regulations will not normally apply. In practice, it is very rare that you will need Building Regulations approval for an observatory, unless it is very big and very close to your boundary.
Householder technical guidance on permitted development:
Government Planning Portal – Outbuildings:
Planning Practice Guidance – Tree Preservation Orders and trees in conservation areas: