Almost 75 per cent of the properties Western Bay building inspectors review for prospective buyers have illegal renovations, which one expert said was "absolutely crazy".
Graeme Blissett and Frans Boucken, directors of Building Surveying Services, have carried out more than 6000 inspections in the Bay of Plenty since 2000.
Mr Blissett has been in the building industry 26 years and has seen "pretty much everything" in the book.
"It's not what have we come across, it's more like what haven't we come across. We've seen all sorts of weird and wonderful things, even dodgy things," he said. "I've seen illegally installed drainpipes, unsafe work, leaky homes, you name it, I've seen it all. I once saw a deck that wobbled like you're in an earthquake. I refused to walk on it.
"[On Tuesday] I visited a place which had blocked drains and the wastewater had collected under the house like a swimming pool and was causing everything to rot. It was crazy."
Of the buildings he inspects, Mr Blissett said close to 70 per cent had some form of "dodgy renovation". "We see a lot of people doing their own DIY who haven't got consent then, in retrospect, realise they should have done it correctly in the first place. Some plainly ignore the fact they have to have building consents and other people don't think it's necessary.
"Some people think by not getting consent they'll save money but, in fact, it can get quite expensive when the job hasn't been done properly in the first place and needs to be fixed or redone."
Tauranga City Council's senior building officer, Brian Swale, said people could face a fine of up to $200,000 under the Building Act for undertaking home renovations without consent.
"A person can face prosecution or a fine infringement notice but we try and resolve all issues without legal action."
Mr Swale is with the council's building monitoring team, which looks into reports of illegal building work. In 26 years in the job, he's seen people do illegal work to create whole households without consent.
"Especially in these economic times we see a lot of second households being created in garages or basements and that's illegal if you don't have consent."
Another common issue was people adding a bedroom and living space in the mezzanine of a commercial building and living there to save money.
"I saw one instance where people were living above a business that manufactured paint. The only way out [of the building] was through the front and if it had a fire they would be toast because there's no way out, especially with hazardous paint thinner and other flammable materials inside."
He's also seen internal enhancements that jeopardised the building's structural safety.
Mr Blissett said many people didn't realise the potential danger they were putting themselves in when doing DIY.
"We see a lot of non-qualified people doing work around their homes all the time. People doing electrical work, plumbing, water-proofing, building showers, even though they're not qualified. It's quite eye-opening and very dangerous.
"People think they can do it by themselves but they don't realise they could be putting themselves in danger or, at least, implicating themselves further down the track when they want to sell their house.
"Most people think they won't get caught but it will catch up with them."
Mr Swale said before any enhancements were made to any home, property owners should visit the council and find out what requires consent. "They can choose to do DIY and even if the work they need to do doesn't need consent or they get an exception, they still need to comply with the building code."
EXAMPLES OF WORK THAT REQUIRES A BUILDING CONSENT:
Structural building - additions, alterations, re-piling, demolition
Plumbing and drainage (except repair and maintenance)
Relocating a building
Installing a woodburner
Installing a commercial air-conditioning system
Retaining walls higher than 1.5m
Fences or walls higher than 2m
All swimming pools and their associated fences
Decks, platforms or bridges more than 1m above ground level
Sheds greater than 10sq m in floor area
EXAMPLES OF WORK THAT DOES NOT REQUIRE A BUILDING CONSENT:
A patio or deck at ground level
Garden trellis less than 2m high
Maintenance such as replacing spouting or a piece of weatherboard
Building a small garden shed (provided it is no closer than its own height to the boundary, is under 10sq m and less than one storey high)
Irrespective of whether or not a building consent is required, all building works must be completed in accordance with the National Building Code.