In reply to katiecat:
Yes there are restrictions building something that large in fact. Generally if you wish to build something in your garden that is further than 5m away from your house is treated as an 'outbuilding', if not it may be treated as an 'extension'. Outhouses can be built without any planning permission (generally) as can extensions if they comply with given parameters. This is known as 'Permitted Development'. You can search for information on this on the Planning Portal online.
It sounds like at 8m you are way out of your Permitted Development rights and that the structure may be treated as an extension, in which case you may need to apply for planning permission. Though I might also mention that if you agree with your neighbours and it is hidden from any public streets, you may get away with it though it will be largely at risk of having to be taken down if spotted by the council or other regulatory body.
As other have mentioned it is worth considering whether there are any covenants on your land or clauses in your leasehold contract (if relevant) that may restrict you from building something like that should you wish to go ahead with applying for permission. In each borough there are exceptions to these general guidelines so I'd recommend checking with your Local Planning Authority.