On the legal side of things, here's some information I've copied from the Web regarding laws in the UK. I have no idea if it's accurate or up to date. If anyone knows I would be interested.
"Electronic Eavesdropping and the (UK) Law
Within the context of law and regulations there are four aspects:
1.The legality of stealing or intercepting information
2.The legality of entering premises to install devices
3.Regulations concerning the operation of equipment and its connection to other equipment
4.The legality of the manufacture, sale and ownership of eavesdropping devices
Laws and Regulations concerning all four aspects differ from country to country. The UK is considered lax in its attitude towards electronic eavesdropping.
Stealing Information
At present, the only legislation making the act of stealing information by electronic eavesdropping subject to criminal law is the 1985 Interception of Telecommunications Act. As the title states, this covers the interception of telecommunications (e.g. telephone calls). The use of equipment to listen into room conversation is outside of its scope according to our interpretation.
Entering Premises
This is covered in some situations by criminal law in respect of Breaking and Entering although those engaged in installation of listening devices argue that the only possible offence is one of trespass. The installation of equipment itself is not an offence except in the case of unlicensed radio transmitters and unapproved telephone attachments.
Operation and Connection of Equipment
In the UK it is an offence to: Operate a radio transmitter that is not type approved (i.e., tested and certificated by the licensing authority) or without a licence - The Wireless Telegraphy Act of 1949 provides for confiscation of equipment and a fine.
The Department of Trade and Industry is responsible for its enforcement. The impracticalities of detecting and tracing a low powered radio transmitter make operation of equipment virtually risk free from this point of view. Connect equipment to the telephone network that is not approved (i.e., tested to specification and certificated by the testing authority).
The operation of tape recorders, mains carrier transmitters, long-distance microphones, optical transmitters is not restricted.
Manufacture, Sale and Ownership
In the UK it is NOT an offence to manufacture, offer for sale or own any eavesdropping equipment. With respect to the equipment designed for attachment to a telephone line, the suppliers are obliged to indicate on the equipment that the equipment is not approved for connection."
On the subject of rooms, I did let rooms in my house at one time, but sadly didn't take full advantage when it came to recording, although I did make some. If anyone knows of a forum to talk about lodgers I have plenty of stories to tell there

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Regarding fantasies, I think it would be fun to have a small office and hire in temps. Another fantasy is to let out a caravan, but I'm not sure how easy it would be to attract the right . Unfortunately, unless I win the Lottery, neither is an option for me.