Little is left hidden in the red light district of Amsterdam.

Here, generally, display is rather more to the point

But, pick your way past the brightly-lit shop windows with their array of human goods and you find something truly hidden – a secret Catholic church created in the loft of a 17th century, canal-side merchant’s house on Oudezijds Voorburgwal.

Our Lord in the Attic, Ons’ Lieve Heer op Solder, has been a church since 1661 – it celebrated its 350th anniversary in May – and a museum since 1888.

It was the main place of worship forAmsterdam’s Catholics during two centuries of repression – following the so-called Alteration, which transferred power in the city to Protestants in 1578, and under which Catholics could worship, but not in places that were recognisably churches from the outside.

Visitor numbers increased dramatically in the early years of this century until, in the year 2005, 90,000 climbed the winding wooden staircases through the remarkably-intact former-home of merchant Jan Hartman to reach the galleried church above: so many that they became a serious problem.

The church was being profoundly damaged by the tramp of feet, the wet raincoats and hot breath of all those pilgrims.

Something had to be done. But what?

Could the damage and decline be reversed?

Would visitor numbers have to be cut?

And, if restoration were to be conducted –exactly what form should it take?

Our Lord in the Attic had been the surreptitious Catholic parish church forAmsterdam’s city centre for over two centuries, until St Nicolas’s church opposite Central Station was dedicated in 1887.

If the years were to be peeled painstaking back, which date should the restoration focus on?

Questions, questions.

A key member of the team that has had to find the answers is Thijs Boers, one of three curators at the museum, and responsible for the conservation effort.

Thijs says: “We had our gut feeling about the damage the visitors were causing, to the fabric, and to the climate in the building, but that was not enough. We needed to know for sure, we needed to get in the expertise to test this, so we could be certain what the threats to the building were – and how to deal with them.”

The museum turned to experts and organisations including the Netherlands Institute for Cultural Heritage, and the Getty Conservation Institute. A commission of experts was established.

Over two years these august bodies assessed the indoor climate, the condition of the collection of art and artefacts, of the building, of the impact of visitors, and also looked at the visitors’ experience.

There was good news early on: the house and church could cope with the numbers that wanted to visit it, if things were managed effectively. Key to that was to purchase the house next door, across the Heintje Hoekssteeg alley, which will become a visitors’ reception area, from which they could be delivered to the house through an underground passage, leaving the worst of the outdoors behind them.

But some of the other fundamentals were not nearly so straightforward. They involved what you might call a holistic approach to the restoration or, as Thijs puts it: “We needed to listen to the history of the house.”

If this were to be a conservation effort that successfully uncovered and sustained the house and the precious church with its commanding18th century altarpiece showing the Baptism of Christ, it had to be done with empathy, and a great deal of research.

Thijs and his colleagues came up with guiding principles that have shaped the Euro10.5m restoration.

One was a radical departure from previous practice.

The house had been treated as a museum; a receptacle for artefacts but,

Thijs says: “We wanted to treat it as an historic house, and a home, with a church inside it.

“For instance, a painting in a particular room might be hung as it would in a museum, for maximum visibility but, from an historic perspective, a painting of that kind might never be placed there. Indeed, it might never have been in that room in a 17th century house inAmsterdam.”

And then there were the stories of the dwellers in the house – or rather houses. Merchant Hartman also owned the two tucked behind it. The church, which holds a congregation of 150, runs the full length of their linked attics.

Thijs says: “We discovered we had three stories we wanted to tell: of the owner Jan Hartman; of the original priest, an Augustinian called Petrus Parmentie; and of the church itself.”

Hartman was an interesting example of social mobility. “He came toAmsterdam, as an apprentice to a baker, from his home of Coesfeld inGermany.

“Later he went into business selling hosiery. He was obviously a pretty good businessman – it was hard to move up – and he later became a trader in linen. You needed connections – family in the widest sense – in order to make your way in the city in those days, and he clearly had those.”

Hartman made a lot of his money from the practice of the city authorities of leasing tax collection rights to private collectors.

His faith was no doubt his main inspiration for building his church but, Thijs points out, it also made business sense.

Because Harman made most of his money from tax collection, his attics – which most merchants crammed with stock – were relatively unused. And he charged the priest rent on the church, and for his accommodation.

The other key consideration that shaped the restoration effort was this:  How do you choose where to go back to?

Thijs answers: “You go back to an ethical point but no further. By that I mean you go to the last period when the building was used for its original purpose – in our case as a church, before it became a museum in 1868.

“To go back much further, we’d have had to demolish the Baroque altar which was installed in the last big changes in the 18th century, and remove the organ, installed in 1749. That wouldn’t be ethical.

“With all of these considerations combined, we narrowed the period of the restoration to between 1800 and 1868.”

Infact they were able to be much more precise; rooting their restoration in a specific year: “We had an inventory for 1862, and accounts from 1850 to 1870s, so the year 1862 as the focus for our restoration looked good.

“And one past priest had interviewed two old ladies who lived in one of the houses behind the church, and could tell him how things were done in the 19th century.”

The work of restoring Our Lord In the Attic to the church it was in 1862 is 90 per cent complete. When it’s done, the monthly Mass will be reinstated. The two houses behind the main dwelling will be finished next year.

Then it just remains for the subterranean passageway to be dug from the new visitors’ centre to the house – which will be completed in 2013, assuming they don’t strike anything historically significant that halts their tunnelling. Thijs doesn’t joke about such things: “InAmsterdam, that can happen.”