Material costs for self-building a double garage

Hello,

our house is to receive a double garage, size 6x7.5m. The floor area ratio is already maxed out so I would first need to inquire whether 6x9 would also be approved. In any case, the garage should have a flat roof and a door. The gate is to be a large sectional door with electric drive and remote control.

Quotes for a prefabricated garage were between 18-20T without the foundations. I then tried to create a material cost breakdown for a self-build and hope for your expert feedback on whether this list is realistic. For the construction itself, I could rely on experienced all-rounders and bricklayers.

==Building application==
Surveyor update of the site plan 200
Building application under the exemption procedure 50

==Foundation slab==
Concrete with strip foundation 1400
Formwork 100
Gravel 1000
Excavator rental, 2 days 160
Reinforcement 200
Vibratory plate rental, 2 days 80

==Walls==
Ytong, 21cm 1500
Mortar 50
==Ceiling==
Concrete 900
Formwork + supports 200
Reinforcement 200
Roofing membrane 120
Aluminum finishing strip 280

==Fittings==
Wastewater 50
Cables and light switch 50

==Gate/Door==
Gate with electric motor 3000
Metal door 200

==Wall design==
Interior plaster 150
Exterior plaster 150
Clinker for the front, 5 sqm 200

==Miscellaneous==
Miscellaneous 200
Tools 300
============
Total approx. 11T

Thanks and best regards, Jens

As far as I know, masonry is usually more expensive than a prefabricated garage. For my construction project, I have received offers for 9x3m garages (so not double) including door, window, electric drive, and everything, ranging from 7-9k euros (excluding foundation, which is always offered to me for about 1k euros), so I can imagine that with some searching, a provider can be found who can offer your shorter double garage for 15k.

Why a concrete ceiling? Put a wooden shed roof on it, you'll save even more. How big should the gate be? We counted 900€ for 3M x 2.75 m. 3k is already set high. Is gravel that expensive where you are? It costs 40€ for 3 tons if you pick it up yourself at the gravel pit. Do you want to concrete the floor or pave it? Do you need a structural calculation for the garage building permit? The exemption procedure doesn't work if you want to build it bigger!

I roughly googled to determine the prices and probably haven't found the cheapest option yet, but rather set the price a bit higher. If you say that some prices are already calculated on the high side, that would confirm my assumption. It should definitely be a concrete ceiling. With double prefabricated garages, there is always the problem that the partition wall has such a frame and the whole thing looks assembled. That is a disadvantage that I don't particularly like.