WC add on to a garden extension

You will require building regs approval for both your extension and any new WC whether or not its within size limits. (i.e. Permitted development). (a conservatory is considered to be a "temporary construction" so does not require planning regs approval.) The new WC will have to be connected to a soil pipe and the overall footprint of the new constructions must be within the permitted space. You may be permitted a larger area with full plans approval from your council.

Any part of a building you alter structurally you require building regs. Evan if it comes under permitted development you still need building regs. Yes you can add a WC to your extension however depending on what direction any underground pipes run you may require a BOS agreement with your local Water authority ( Build Over Sewer Agreement ) If the run is connected to a live run which means other household sewerage runs through it I know with Welsh Water they are held responsible for its upkeep. If its not live its the responsibility of the property owner.

Different areas, different councils, different roles. Sometimes you may be able to do it, and sometimes not, or sometimes within some strict specific regulations. Personally I can confirmed we've had them build in London ( within M25 ) even in the conservation areas ( listed buildings are very strict and unlikely to have this kind of out-builds but you never know until you ask and check ). Your architect, best if its a local person, ( doesn't mean not local architect won't be able to deal with this ) is the best person, to check it with the council ( this can be also done by yourself by calling your planing department and talking to one of the planning officers in your council who will be able to clarify all unknowns ). It also depends on some other aspects and may involve more work then you think and price will vary. For example - You can prevent contacting Thames Water and Build Over Agreement if such is required by using macerator and connecting to the existing internal pipes within your property. You can also have a kind of garden shed made off timber insulated frame which is often common solution. Designers and planners will be best to act on your behalf as roles are changing and they are/should be your first contact as some of the builders may not have required knowledge so you shouldn't relay on them and for your safety this would be the best rout.

Whilst a conservatory doesn’t always require planning permission as it doesn’t have a solid roof, as such. However, consideration should be given if an extension or extensions have been added to the property as this can affect whether planning permission is needed even for a conservatory. My advice would be to contact planning who will be able to advise. With regards to the construction of an extension, whether to replace a conservatory or otherwise, it will require a submission to building regulations. Though sometimes construction might be carried out under a building notice, Building Control are very approachable and will provide excellent advise generally speaking