I urgently need you to check these sentences, please. Thank you. I need to know if the passive forms are correct and possible.

1) He's going to do the washing after lunch.
The washing is going to be done after lunch.
2) He's going to book a seat for tomorrow's concert. A seat for tomorrow's concert is going to be booked.
3) They were renovating the flat when I was there last week. The flat was being renovated when I was there..
4)The cleaner is polishing all the floor with a new wax polish. The floor is being polished with..
5) Who fed the baby? Who was the baby fed by?
6) What do the serve the custard with? What is the custard served with?
7) Did Jennifer organise a concert for charity? Was a concert for charity organised by Jennifer?
8)Somebody is cutting the grass in the garden. The grass in the garden is being cut

1. Do you mean washing clothes (doing the laundry) or washing dishes (doing the dishes)?

2. OK (first sentence is better because it's in active voice)

3. OK (same as #2)

4. OK (same as #2)

5. OK (first sentence is MUCH better...)

6. Use the second sentence.

7. See #2.

8. See #2.

The passives are correctly phrased, but the actives are preferable.

Here are my suggestions for the passive forms of the sentences you provided:

1) He's going to do the washing after lunch.
Passive: The washing is going to be done after lunch. (The active form is already in the passive voice, so no changes are needed.)

2) He's going to book a seat for tomorrow's concert.
Passive: A seat for tomorrow's concert is going to be booked.

3) They were renovating the flat when I was there last week.
Passive: The flat was being renovated when I was there.

4) The cleaner is polishing all the floor with a new wax polish.
Passive: The floor is being polished with a new wax polish.

5) Who fed the baby?
Passive: Who was the baby fed by? (No changes are needed for this sentence.)

6) What do they serve the custard with?
Passive: What is the custard served with?

7) Did Jennifer organize a concert for charity?
Passive: Was a concert for charity organized by Jennifer?

8) Somebody is cutting the grass in the garden.
Passive: The grass in the garden is being cut.

Please note that in some cases, the passive form might sound less natural than the active form. It's always a good idea to consider the context and choose the most appropriate sentence structure.