Conversion of an existing extention into a full wetroom

[noob_homeowner]

Hi,

Just after a bit of advice, I'm about to complete a purchase on a terrace house and am after some information about what I can change in it.

There is an extension leading from the kitchen. Kitchen has a bathroom above it, the extension is a single storey, flat roof, structure that incorporates an inner lobby, inside toilet and a door to the back yard. Once in the back garden there is also access to an outside storage space that makes up the remainder of the extension via a secured doorway. Internally the lobby and toilet are separated from the storage area by a brick wall.

As follows: http://i.imgur.com/pnKCD.png

We want to block both the door leading to the back yard and the door to the storage space, remove the internal brick wall and allow the bathroom to cover the entire existing internal space.

As follows: http://imgur.com/KMgWE.png

Note - I forgot to draw it but there is also a small window already above the toilet.

I just have a few questions about this -

- Does this require planning permission?
- Will this have to comply to building regulation?
- Will we need a completion certificate upon completion or get a building warrant of some kind?
- Do we have to submit plans and/or get prior approval before commencing work.
- Is this expensive (the admin not the parts and labour for the construction/installation)?

I believe there is no longer an issue with the whole double door between the toilet and kitchen, provided there is a place to wash hands in the toilet. Is this the case?

Can anyone provide a brief step by step process to getting this done assuming no work has yet started? Not sure how this is supposed to be managed as a job - order of tasks etc. Failing this, if you can point me in the right direction so I can find out for myself (somewhere reliable) then that would also be great.

Many thanks in advance for your help, any info will be gratefully received.

Cheers

J