On average how long will a loft conversion take to complete?

I am having the insulation in my loft topped up for free by British Gas. They are putting an extra 8" in on top of the insulation that already sits in the loft and lies just below the joists. I assume that the joists will be covered with the fresh insulation but having totally cleared out my loft I want to board it out properly after British Gas have finished. Can the boarding go on top of the insulation or will the joists need raising?

I'm looking at a conversion that includes a new staircase for access and an on suite in the velux type of conversion

We bought a 1905 terraced house, which was sold to us as a 3 bed + bonus room. The bonus room referred to the loft which had been converted years and years ago by one of the previous owners. It was sold as a bonus room because there was no record or document (building regs certificate, etc.) for this loft conversion.
The loft conversion currently consists of the following:
spiral staircase (very space saving, but the balustrade has become very wobbly),
some electric work for lighting and also electric heaters,
some insulation (most likely not up to scratch Vs current building regs),
floor: not sure how to check if this was properly done,
Dormer window looking to the back.

We would like to refurbish the loft (fix stairs to make them more sturdy, fit double glazing for dormer window, fit new velux opposite dormer window to bring in more light, fit built-in cupboard).

Ultimately we want to use the loft as a study/play room. Do study/play rooms count as habitable rooms in which case we would need to re-do the loft conversion entirely with up-to-date building regs? Or can we get away with doing the above refurbishment without building regs?

我正打算联系市议会的建筑控制部门，但也想从专业商人那里得到一些建议。很抱歉问了这么长的问题……

=======Update 23/04/11===========
Thanks all for the replies so far. Had further thoughts: so ok we could do the refurbishment (fix spiral stairs, replace wooden dormer window by uPVC double glazing, etc.) without need for building regs, BUT when we come to sell our house, potential buyers could be deterred by the fact that building regs are still missing.
So I think we have 2 options:
option A) have a surveyor come to our house to do a full structural survey of the loft to ascertain the roof structure is sound and the floor is strong enough to be used as bonus room. Providing there is nothing drastically wrong, proceed with refurbishment. Pros: cheaper and quicker job. Cons: building regs still missing which could be a problem when we come to sell our house, but at least we would have a full structural survey to show...
option B) do a full refurbishment to bring loft conversion to current building regs. Pros: peace of mind that structurally all is sound, easy resale of property. Cons: will take more time and cost a lot more than we had planned/budgeted
If anyone has further inputs/comments, they would be greatly appreciated. Thanks

velux和dormer的区别是什么?一个比另一个好吗?

I have recently had my loft insultated (double layer) and i was told that if i wanted to board it (use for basic storage - Christmas deco...) I would also need to raise the joists then board.

I have had the opposite advice from a trusted builder and have been told just to board directly onto of the double insultaion with ply or loft boards using the existing joists.

What is the right way, and why?

Currently have a partially boarded loft. The previous owner nailed the boards directly to the joists instead of using stilts. We now want the floor fully boarded, should we remove the original boards and do the whole thing again using stilts? I understand there are issues with insulation height but could boarding directly onto joists also cause issues with damp?

Thanks

Hi. Not looking for any estimates on this, just an idea of the work involved which we'd need to consider.

We have an existing loft conversion - two bedrooms, one with a dormer, one with a velux window. The velux window needs repairing and now we're thinking about it my husband has expressed an interest in installing a dormer window instead, to open out more space in the room.

On average, how big a job is this in terms of steps involved, disruption, and time taken (quite apart from the cost, which clearly isn't something we can go into here?). I'm thinking

- drawing up plans
- permission?
- scaffolding and permission from neighbours to put it up
- removing existing window, putting a big hole in the roof (!), adding in walls and roof, installing window...
- plastering and decorating

Can anyone correct / expand on this, and let me know what it's like living through such a job and how long it can take, from experience?

Thanks in advance.