12" ft sounds excessive so ring your local planning department and they will inform of any rules re size, type of structure permitted etc.

I was going to say the same. They might need planning permission for some so called children's play houses.

Garden structures which are within 2 metres of a boundary and over 2.5m high (8'2") generally require planning permission. It also depends if it is near a public path, conservation area etc. Certainly worth calling the local authority to check as per the above.

If you are on good terms with them and wish to avoid long term animosity between you both, suggest nicely that they check the local ordinances before they continue. Tell them you are thinking of them with their best interest in mind and you wouldn't like to see them hit with violations and hefty fines. If they blow you off then say no more and quietly wait for them to complete their project. Once completed then contact the local authorities with your concerns and to be sure, the building inspector will take it from there. In the USA most property is on a satellite image grid which is monitored by a computer data base. If any changes occur on the individual's property grid, such as a new structure or a change in an existing structure a red flag goes up and the local ordinance authorities go out to investigate--this is our Brave New World friends.