I want to extend over 3 meter's deep and have semi detached house

You can go past 3m on any development with planning permission. Below and including 3m just means you don't need planning and only need to involve building regulations. I wouldn't chance it without that personally

why you asking us ,we are builders not planning officers ,submit the drawings and calcs and see ,varies from county to county

all the information you need is on the "planning portal" i would also recommend you notify the council and request confirmation for your proposed works before going any further. it would be a pity if it had to be changed afterwards.

All good answers above, although getting to speak to a planning officer is difficult these days. The key issue here is the right of light to neighbouring properties. You need to take a line at 45 degrees angle from the middle of the windows to living rooms and bedrooms on neighbouring houses, if your extension is likely to cut through the line, then they likely to object. It seems though they are all doing the same, so probably not an issue. If you are building within 2m of a boundary, you will need planning permission. The depth of the extension away from the original wall will affect the type of roof you can have. There is usually around 1.2m between the top of the ground floor windows, and the cill of the window above, and the roof structure of the extension needs to fit within the band. Generally around 3m will allow for a lean-to roof, with concrete tiles on, the further away you get the shallower the pitch is or the flatter the roof becomes. Andrew

I'd first speak to the duty officer at your local planning office. If you describe exactly what you want to do, they may be able to give you a decision in principle over the phone.