Permitted development

[veuveo]

Would I be able to add on to extend the utility room? The current utility was part of the original garage and I could either extend it back into the garage or add on.
House has already had big extension.
Would be by about 3m

There is a limit to pd which takes account of previous extensions. You would need to check your council's website to see if you have already used up your rights.

Extensions (including previous extensions) and other buildings must not exceed 50% of the total area of land around the original house
This?
Bit vague.
There's nowhere on the website that lets me know how much of the 50% my house has taken.
Is that what you mean? Or do I contact the council directly?

Yes and there are other restrictions ie if extension would be to side of house (am guessing it would be if it's behind garage). Think you'll need planning permission.

It is usually calculated against the original square footage of the house. Currently many areas also have a larger permitted development policy which requires approval but is a less time consuming process than planning permission.

The Permitted Development allowances are always measured from the house as originally built (or as it stood on 1st July 1948, if it is older than that).

Beyond that, refer to the Permitted Development Technical Guidance:
www.gov.uk/government/uploads/system/uploads/attachment_data/file/606669/170405_Householder_Technical_Guidance__-April_2017_FINAL.pdf

Superficially, it sounds like what you're suggesting could well fall within the scope of Permitted Development, but there are an awful lot of 'ifs' and 'buts', so best to check in detail against the Technical Guide, and to seek the advice of your local Planning authority if you are in doubt.

The extension would be at the back of the house
The utility was originally part of the garage.
So I could extend out or take back part of the garage. Which would probably be easier but I don't want to mess around with positioning of the toilet.

Sometimes permitted development rights have been completely removed, which means you have to have full planning permission for even a porch, this does tend to be on newer properties built in the last 30 years or so, check before you build anything.