Customers of a water company are being charged hundreds of pounds more than people in other parts of the country, it has been claimed.

Following complaints from homeowners, consumer magazine Which? launched an investigation into how much people are charged for “build overs”.

This is an agreement to allow a home extension or other building to be built on top of existing water mains and sewers.

Which? asked all 23 UK water companies how much they would charge for a typical single-story extension within three meters or directly over a public sewer.

While Yorkshire asks for just £30, Southern Water, which supplies the vast majority of households in Sussex, demands a whopping £812, with the rest charging somewhere in between.

Peter Vicary-Smith, chief executive of Which? said: “It appears that some homeowners face a postcode lottery when it comes to the fees charged by water companies for straightforward works.

"There's no clear explanation why [the charges should be different] - it seems to come down to the luck of the draw.

“We'll be sending our findings to Ofwat and the Consumer Council for Water, as we think this is unacceptable.”

A spokeswoman for Southern Water said the different charges might relate to the amount of work each company does.

She said: “Our fee is a one-off, flat rate that includes the application and survey and all areas of work after the application has been made - from liaison with the local authority and applicant, through to the conclusion of the agreement.

“We refund the fee if the sewer is more than three metres away; it’s clear from the out-set the application cannot progress; we are unable, for technical reasons, to carry out the survey.

“Other companies may charge on a different basis, which will affect any comparison between companies.”

A spokesman for the Consumer Council for Water referred the matter to Ofwat, who told The Argus it had not powers of regulation over “build over” charges.