The multipolygon is not needed and you can eliminate it.
You have to create a point of contact where the paths intersect

way/1218545045 with way/1218545046
way/1218545043 with way/1218545044

This way you will be able to create the "outline" of the building, to be included in the building report as an "outline"

For the helipad not having specified the thickness of the plot, in JOSM, it seems to start from the ground.

Hi,
You are mapping a building for 3D rendering.
You have rightly used a building type relationship to group the parts of the building.
You created a relation type multipolygon to group the parts of the ground floor and then inserted it into the building relation as an "outline" member.
This relationship is not necessary for two reasons:
1 - The parts of the building are already found inserted in the building relation.
2 - The multipolygon does not represent the perimeter of the building projected on the ground (Top View).

As you can see, the absence of the outer perimeter, does not allow the proper 2D rendering of the building
relation/16603276#map=18/47.49661/9.74649

At present, even the 3D rendering does not reflect the building outline
https://demo.f4map.com/#lat=47.4966370&lon=9.7454901&zoom=20&camera.theta=67.108&camera.phi=-1.432

You can also notice the lack of the minimum level on the part representing the helicopter pad.

If there are rules it seems to me that they should be applied.
If they are wrong it is equally true that they should be changed.

OSMI verifies compliance with the MP construction rules.
Then the MP rules described in osm.wiki/Relation:multipolygon should be changed.
In any case for 3D mapping of buildings as in this case, the rules detailed in
osm.wiki/Simple_3D_Buildings

In the relation
relation/16564239 Version 4
data are not duplicated, because they already exist on the perimeter
way/1218755234
which delimits all parts present in the building report.
In this case, it is advisable to apply the rules of "Simple 3D Buildings" that avoid reports in OSMI for incorrect use of MP rules

No redundant multipolygon is created.
Only one building multipolygon is used to join the various parts.
On the outer perimeter, of the building, the common data of the building is entered.
You don't need the "Relations of type multipolygon" multipolygon in fact I transform it to "multipolygon tagged with building=*"
osm.wiki/Simple_3D_Buildings

Hi,
The multipolygon (MP) relationship is generally used for areas.
osm.wiki/Relation:multipolygon
For mapping buildings in 3D, the use of building relationships is appropriate
osm.wiki/Simple_3D_Buildings
Use of the buildings relation facilitates rendering of buildings without flagging in OSMI
https://demo.f4map.com/#lat=47.4957691&lon=9.7460920&zoom=21&camera.theta=65.103&camera.phi=-51.462
https://tools.geofabrik.de/osmi/?view=areas&lon=9.74663&lat=47.49629&zoom=18&baselayer=Geofabrik%20Standard&opacity=0.19&overlays=duplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways%2Cduplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways
Greetings

Now they are connected to the roof
way/15805226
OSMI should not report problems at the multipolygon

Ciao,
Avendo rimosso il buco centrale, il multipoligono è diventato un semplice poligono.
https://osmose.openstreetmap.fr/en/issue/6f626144-7fe6-7728-9285-bae2a67b68de
Andrebbe quindi eliminata la relazione. Puoi anche considerare di inserire il tag "garden:style=flower_garden"

Ciao Đuro Jiří,
Ho modificato la tua mappatura in quanto la relazione relation/16087959 violava le regole dei multipoligoni osm.wiki/IT:Relation:multipolygon.
Naturalmente sono stato tratto in inganno dalle foto aeree ed ho mappato la superficie come coperta da ghiaia.
Dalla foto allegata appare che la superficie mappata è una aiuola fiorita senza "il buco centrale" pertanto sembra più appropiato l'uso del tag "landuse=flowerbed" senza l'annello interno.
Cosa ne pensi?

I merged the multipolygons of the rocks, removing the relationship with fewer members and the segments no longer needed. Updated the resulting relationship (one outer and two inner areas).

Hello,
Today OSMI
https://tools.geofabrik.de/osmi/?view=areas&lon=10.52738&lat=46.15749&zoom=15&baselayer=Geofabrik%20Standard&opacity=0.63&overlays=duplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways%2Cduplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways
Reports problems at the bare rocks multipolygon.
That area is described by two multipolygons
relation/12991396
relation/12991394
which could be unified by removing the segments
way/1184255723#map=17/46.15989/10.52399
way/1184255725#map=15/46.1587/10.5231
and the straight part of the segment
way/1184255724

Hi,
Simply OSMI was reporting the presence of 2 nodes in the same location.
By doing the Merge between nodes one was removed as it was redundant.
Vedi https://resultmaps.neis-one.org/osm-change-viz?c=136818872#21/47.35353/8.51060
I see that you entered the area as "place"
way/117778756
In this case JOSM reports the omission of the name.
Regards

Salve,
Non capisco il motivo di questo commento.
Ho eliminato un percorso che ho inserito per chiudere un multipoligono, che in seguto al ripristino dei dati è risultato inutile.
Non dovevo farlo?

Stammi bene

The glacier area "Vedretta del Mandrone" was included by @Edoardo%20Zanotti with way/888903146 in the multipolygon relation/3320630#map=13/46.1616/10.5284. This change resulted in an OSMI report for duplicate segments, which I corrected without changing the extent of the area.
If you know the boundary of this glacier edit the incorrect portion.

Ciao,
Mi permetto di segnalarti i problemi rilevati da OSMI in merito ai tuoi recenti inserimenti di dati.
https://tools.geofabrik.de/osmi/?view=areas&lon=9.66926&lat=45.84349&zoom=16&baselayer=Geofabrik%20Standard&opacity=0.20&overlays=duplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways%2Cduplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways
Tra le altre cose hai creato la relazione relation/14962927 per delimitare/unire aree che non risultano omogenee utilizzando le etichette:
landuse=religious
name=Ghiaia
type=multipolygon
Immagino che le aree, ragruppate insieme,non abbiano niente di religioso e il loro nome non sia Ghiaia.
Ritengo pertanto sia più appropiato l'uso di:
surface=fine_gravel
escludendo, naturalmente dalla relazione, le areee con superficie di tipo diverso.
Le regole per la creazione dei multipoligoni le trovi quì osm.wiki/Relation:multipolygon
OSMI segnala anche che il perimetro della relazione non è chiuso e ci sono sovrapposizini di aree.
Per evitare questi inconvenienti è opportuno non ignorare gli avvertimenti dell'editor in fase di caricamento dei dati.
In loco hai anche creato una relazione che ragruppate tutte le aree pedonali (marciapiedi), non compredo la motivazione di questa scelta, che oltre tutto, rende difficile la modifica/manutezione visto l'estesione e il numero dei membri.

Served by a road that is not propitiously agricultural are a restaurant, a church, a castle, and a farm. I am reasonably convinced that patrons can access them with their own vehicles (motocar=destination).

Hallo,
OSMI berichtet derzeit:
https://tools.geofabrik.de/osmi/?view=areas&lon=16.73389&lat=47.95795&zoom=18&baselayer=Geofabrik%20Standard&opacity=0.40&overlays=duplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_no
Nicht alle Probleme gelöst
Grüße

Ciao,
Vecchia espressione inglese DIRFT: Do it right first time every time (Fallo bene la prima volta ogni volta.)

Indubbiamente si tratta di una costruzione sul mare e non di un edificio.

Il ragruppamento delle parti di questa costruzione in una relazione multipoligono non è indicata perchè ne verebbero violate le regole. La possibile soluzione, per ragruppare le parti, è quella di usare una relazione building (non edificio ma costruzione). Come è evidente la soluzione adottata ha eliminato la segnalazione di OSMI, pertanto non la definirei sbagliata.

La descrizione della struttura è incompleta da ben 3 anni, evidentemente ne eri a conoscenza, ma non hai cercato di migliorarla come specificato più sopra, potevi benissimo aggiungere man_made=pier alle sue parti con gli opportuni tag.