3m in length x existing width of side return. If your side return is say 6m long, you would be left with a 3m long side return.
If your house is L-shaped in plan, i.e has a rear block, you can also extend out 3m from the rear wall of this block, again maintaining the existing width of the block.
If you extended the side return and rear block, you would end up with a 6m side return, the whole thing having shunted forward by 3m. Hope that makes sense...these things are easier to draw!
If you live in a flat, PD regs don't apply and you would need to apply for planning permission regardless how far you wanted to extend.