Chairman of the Council, Councillor Mel Starkey, raised a flag and unveiled a plaque to mark the completion of the external fabric of the building and the removal of the scaffolding.
Leader of the Council, Councillor Reg Shore, said:
"The fabric of the building is now complete and we are 60% there with the electrical work and plastering which means we are on target to occupy our new base in November.
"Our new offices will allow us to operate much more efficiently, and, equally importantly, it opens up our existing premises at the Guildhall, Cross Street and Spital Terrace for new development which will continue to regeneration of Gainsborough.
"We are already in talks about these sites and I am confident that I will be able to make exciting announcements for the town during the next few weeks."
The new headquarters occupies an important site at the entrance to the hugely successful £39 million Marshall's Yard office, retail and leisure development.
The former engineering complex has been transformed by developers Dransfield Properties and Prospect Estates. Managing Director Mark Dransfield said he was delighted that the Council headquarters was now nearing completion.
"This has been an exciting project which has delivered High Street names like Marks and Spencer Simply Food, JJB, Halfords, Costa Coffee and many others to the town. The final piece of the jigsaw is the office accommodation for West Lindsey which is the icing on the cake of what we believe is a first-class development," said Mr Dransfield.
Deputy Leader of the Council, Councillor Tom Brown is looking forward to moving to the new headquarters which has environmentally friendly features that make it a unique public building in the East Midlands.
Solar energy will be collected from panels installed on the roof and rainwater will be harvested and stored in tanks below ground that will be used to flush all the toilets in the building.
Natural ventilation will be achieved by windows that open and close automatically and provide night cooling of the building to reduce the need for air conditioning. There is also a smart lighting system which will turn off or dim the lights according to how bright the sun is or if the room is unoccupied.
The building will be highly insulated to reduce the need for heating in the winter and prevent excessive heat gain in the summer. All appliances will be the A rated to reduce the amount of electricity needed to run them.
Councillor Brown said:
"The 11 k Wp solar power units that have been installed have a life expectancy of up to 60 years and will save 5,000 kg of CO2 emissions a year.
"The system will raise everyone's awareness of environmental issues because there will be a real time display panel in our reception area telling visitors how much power is being regenerated at any given time, the total number of kilowatts generated so far, and how many kgs of CO2 emissions have been saved.
"This is tangible evidence of the Council's commitment to the Nottingham Agreement designed to protect the environment."
Fellow Deputy Leader, Councillor Pat O'Connor said that the new building – on four floors and providing 3,498 square metres of space – will make savings of nearly £60,000 a year compared with the existing accommodation.
Councillor O'Connor said:
"We currently pay £55,000 a year for energy and water at the Guildhall and at our Lord Street and Cross Street offices. This bill will be slashed to £42,000 a year at our new premises.
"On top of this we are spending about £25,000 a year on maintenance of our current premises. Over five years this equates to £125,000 which compares with our estimate of £40,000 for the new building.
"When you add all this together it means that the new building will save us £17,000 a year on maintenance and service costs," said Councillor O'Connor.
"There will also be savings on multiple line rentals for alarm systems, service and cleaning contracts and our business rates will reduce substantially to produce at least £40,000 in revenue savings," he added.