Just going through buying a Taylor Wimpey house and can offer my experience in these......you will not be able to change the plans. We wanted to add an extra window onto an end gable wall so that we had natural light coming into the landing, but this was turned down as they have to go back to the planner to make nay ammends and this takes time and costs money, all of which Taylor Wimpey do not want.
They are (and this is in the words of the site manager and sales woman) 'bulk housing producers and do not do bespoke'
As for extras, they do add up. The basic stuff that the houses come with is just the very basic. We have added £6665 of extras to ours and all that was really was upgrade to the bog standard kitchen, carpeting for upstairs and stairs and then a few extra sockets and lights. To put a light and switch under the stairs was £60......an outside socket £60 and the LED downlights were £70 each).
Yes they are pricey, but weighed up the fact that due to the construction of the house (all walls and ceiling voides are rammed with insulation and sound deadeing materials) it would be a complete mare for me to put in the extras, plus all the make good work, so better for them to carry it out during the install.
Finally, if you are thinking of getting some structured cabling run in during the build forget it. I asked for this and they again said no as it was not an extra they provide. When I said I would provide the cabling they again said no as they did not know the providence and conformity of the cabling. Will be putting it in when I lay the flooring...will be running it in gaps between the underlay to required points. Would have been very handy having it buried in the walls, but hey ho. You may get lucky and have a different manager with different views.....if you do let me know so I can goi back to them with this and get them to change their ways...
Good luck with the sale and move