The properties built on double plots

Hello everybody.

I have some questions. Would you please help me?

What is a property built on double plots? Does it mean it is built on two pieces of land?

What does demanding mean ' in her clear slightly demanding voice'?

And the last question, Issac says from the side of his mouth. Does it mean talking slowly?

I follow Rowan’s eyes, taking in the properties, some of which have been built on double plots. Lots have been extended to three storeys high, too, and most of them have electric gates and security keypads. The tall gates are open at the entrance to the property we’re passing and we gawk up the driveway.

‘I wonder who lives in these massive houses, Dad,’ Rowan muses.

‘Looks like we’re about to find out,’ Isaac says from the side of his mouth, as a girl around Rowan’s age with a brown bob and large brown eyes hares to the front gate, followed by a smiling woman with the same hairstyle and eyes. ‘Are you the new people at number fifty-four?’ the little girl asks. She looks at Rowan without waiting for an answer. ‘I saw you looking out of your bedroom window. My name’s Aisha, what’s yours?’Aisha says in her clear slightly demanding voice'?

Thanks so much.