1.The necessity of repairing the gallery (or shopping arcade????) arouse due to its leakiness, improper falls of the ground which contributed to the leakage of precipitation water through the floor to the underground garages.
2. On the basis of the technologcal evaluation of the gallery Y company carried out an offer campaign thanks to which Z comapny's technology was chosen as well as the indicated by Z company for implementation Idzik P company company with its registered office in Paris
3. Technological project for sealing the terrace around the building was drawn up in 2005
4. Y company signed an agreement with Z company concerning extensive renovation of the gallery outside. It was to be based on the technology worked out by Z company and on X comapny's guidelines. The flate rate of the works was estimated at xxx
Answers:
1. The necessity of repairing the gallery (or shopping arcade) arose due to leaks and the hazardous slope of the flooring which contributed to the leakage of water through the floor to the underground garages.
* you can use either gallery (more general) or shopping arcade. ‘Improper’ may mean something that can be hazardous. ‘Falls of the ground’ can be a descent or slope. Leave out the word ‘precipitation’ since you are really talking about water seepage and not the quantity of water deposited at the place.
2. On the basis of the technological evaluation of the gallery, Y Company offered to carry out a campaign. But thanks to this, Z Company's technology was selected as the better choice, as indicated by Z Company’s implementation of Idzik P Company which has its registered office in Paris.
* when you say ‘carried out’ it means it has already been done, but the campaign has just been ‘offered’ which means it is yet to be done. This was confusing for me so I rephrased the sentence. You’ll be choosing what’s the best answer for you anyway. I’m just trying to help out. I also made two sentences out of the original one long sentence. This way, the idea of the paragraph is more presentably delivered rather than just tumbling all the words together.
3. Technological project for sealing the terrace around the building was drawn up in 2005.
* this is okay. Just added a punctuation (period) at the end of the sentence. But you may also try: Technological project for sealing the terrace located around the building was drawn up in 2005.
4. Y Company signed an agreement with Z Company concerning extensive renovation of the outside gallery. It was based on the technology developed by Z Company and on X Company's guidelines. The flat rate of the work was estimated at xxx.
* change ‘gallery outside’ to ‘outside gallery’ or even ‘external gallery’ which are more correct since it is more descriptive than operative. Drop the ‘to be’ in the second sentence since it is unnecessary. The word "developed" is more appropriate than 'worked out'. The word ‘work’ can stand as a plural form in itself unless pertaining to grouped objects.