When you are dealing with a stiuation such as yours it may well be as well to asume the worst (not nice I know) as there is a lot of very important structural work involved in the floor, even if you just want a roof window (velux) and no dormer. Add to the mix sound and insulation work, not to mention fire regulations, all of which you need "Building Regulations" for and as "boy boy" said this has to be payed for, but you will be legal, which is important for any future sale of the property.
So cost: It does depend on your location in the UK, but if you work on the fact that most of the work that exists is scraped (sorry) the cost can range from £12,000 to £30,000.
My best advice is to get a reputable national company to carry out a complete survey for you, which will mean lifting your flooring of course, but it will be worth it longer term. If you need more information on the best steps to take there's a free report here, or contact the help desk
You say you live in a Terraced house so you will also need a party wall agreement, a surveyor will do this (of course a cost involved) This may seem like a lot of work and expence, but a (LEGAL) loft conversion is the number one home improvent to add real value, the major building societies report as much as 23% can be added to your homes value.
Hope this has helped, please don't go down the road of cutting corners with this, it is and always will be false economy.
One last point, last week in Nottingham there was a fire in an (ILEGAL) loft conversion and the fire fighters coudln't gain acces to deal with it, fortunatly no one was injured. Would you be prepared to put your family at risk?